strategic flood risk assessment - midsussex.gov.ukthe sfra has been used help inform the content of...

Strategic Flood Risk Assessment

Level 1

June 2015

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List of Contents

Glossary and Abbreviations ................................................................................... 5

1 Introduction ...................................................................................................... 6

1.1 Purpose of this report .................................................................................. 6

1.2 Mid Sussex District ...................................................................................... 7

1.3 The Mid Sussex Local Development Framework ........................................ 8

2 Flood Risk ....................................................................................................... 11

2.1 What is flood risk? ..................................................................................... 11

2.2 Sources of flood risk .................................................................................. 11

2.3 Implications of flood risk ............................................................................ 13

2.4 The Strategic Flood Risk Assessment and Sequential Approach .............. 13

2.5 Neighbourhood Plans and Neighbourhood Development Orders .............. 16

2.6 Site specific Flood Risk Assessment ......................................................... 16

2.7 The impact of climate change on flood risk ............................................... 19

3 Methodology and findings of the SFRA ........................................................ 22

3.1 Introduction ............................................................................................... 22

3.2 Coarse Assessment .................................................................................. 24

3.3 Other steps taken to obtain information on flood risk ................................. 25

3.4 Have all hazards been sufficiently defined?............................................... 25

3.5 Overview of flood risk in the district ........................................................... 25

4 Outcomes ........................................................................................................ 27

4.1 Implications for the District Plan 2014 - 2031 ............................................ 27

4.2 Advice on the preparation of flood risk assessments for allocated

development sites ............................................................................................ 28

4.3 Assessment of the implications of climate change for flood risk at allocated

development sites ............................................................................................ 29

4.4 Advice on sustainable drainage techniques for managing surface water run-

off at development sites .................................................................................... 32

4.5 Recommendations about the identification of critical drainage areas and the

potential need for surface water management plans ........................................ 34

4.6 Areas at risk from other sources of flooding .............................................. 35

4.7 Recommendations for Development Management purposes .................... 36

4.8 Recommendations to inform future policy ................................................. 37

4.9 Mapping .................................................................................................... 38

4.10 Flood risk management measures ............................................................ 38

4.11 Future use of the SFRA............................................................................. 39

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5 Limitations, restrictions and statements of accuracy ................................. 40

Appendix A – Coarse Assessment data sources ................................................ 41

Appendix B - Flood improvement/alleviation measures undertaken by the

District Council ...................................................................................................... 63

Appendix C – Flood Risk Assessment and Sustainable Drainage Systems

guidance for specific areas within the District .................................................... 64

Appendix D – Flood Risk Vulnerability Classification ........................................ 70

Figures and Tables

Figure 1 - Map of Mid Sussex District, identifying the main settlements and transport

corridors .................................................................................................................... 8

Figure 2 - Climate change implications at land to the east of Burgess Hill at Kings

Way ......................................................................................................................... 30

Figure 3 - Climate change implications at land to the north and north-west of Burgess

Hill ........................................................................................................................... 32

Table 1 - Housing Requirement for Mid Sussex 2014-2031 ..................................... 10

Table 2 – Flood risk probabilities ............................................................................. 11

Table 3 - Categories and causes of flooding ........................................................... 11

Table 4 - Flood Zones ............................................................................................. 14

Table 5 – Overview of Mid Sussex flood risk characteristics .................................... 25

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Glossary and Abbreviations

The Agency The Environment Agency

CFMP Catchment Flood Management Plan

The District Council Mid Sussex District Council

DPD Development Plan Document

FRA Flood Risk Assessment

FWA’s Flood Warning Areas

GIS Geographic Information System

LDF Local Development Framework

LLFA Lead Local Flood Authority (West Sussex County Council)

LPA Local Planning Authority

MSDC Mid Sussex District Council

NPPF National Planning Policy Framework

OS Ordnance Survey

PPS Planning Policy Statement

PPG Planning Practice Guidance

SFRA Strategic Flood Risk Assessment

SPD Supplementary Planning Document

SuDS Sustainable Drainage Systems

SWMP Surface Water Management Plan

WSCC West Sussex County Council

WSF&RS West Sussex Fire and Rescue Service

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1 Introduction

1.1 Purpose of this report

This report presents the findings of the Strategic Flood Risk Assessment (SFRA) for

Mid Sussex, which has been undertaken by Mid Sussex District Council. This SFRA

has been prepared in accordance with the National Planning Policy Framework

(NPPF) and has followed guidance and advice provided by the Environment Agency,

West Sussex County Council, the District Council’s Drainage Team and set out in

National Planning Practice Guidance (NPPG). This SFRA was initially prepared in

2008 and has been reviewed and updated in 2015.

The aim of planning policy on flood risk is to protect people and property from

flooding. The SFRA should therefore enable decision makers to fully understand the

flood risk in the area by identifying the areas that are at risk from all forms of flooding,

now and in the future, taking account of the impacts that climate change and

development will have on flood risk in Mid Sussex. This enables the District Council

to inform the scope of the Sustainability Appraisal for consultation, develop policies

that seek to reduce and manage flood risk and identify where development can be

located in areas with a low probability of flooding. The SFRA will inform other work

including site-specific flood risk assessments and neighbourhood planning.

This SFRA forms part of the evidence used in the development of the Mid Sussex

District Plan 2014 – 2031 which will provide a framework, vision and delivery strategy

for development in the District and has informed the Sequential Test and

Sustainability Appraisals (incorporating the Strategic Environmental Assessment

(SEA)). A draft of the 2008 SFRA was also used as part of the evidence base in the

production of the Small Scale Housing Allocations Development Plan Document.

The District Council’s SFRA will continue to be updated and used in the development

of other Local Development Documents, which the District Council will prepare,

Neighbourhood Plans, for assistance in the determination of planning applications

and for emergency planning purposes.

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1.2 Mid Sussex District

Mid Sussex District is located between Crawley and Brighton, in the County of West

Sussex. The District covers an area of almost 130 square miles and has three main

towns; Burgess Hill, Haywards Heath and East Grinstead. The remainder of the

District is largely rural in character with 25 villages and many small hamlets. Some of

these villages are considered to act as a local service centre for surrounding rural

areas, in that they contain a range of community services and facilities, and are

therefore significant in size with a resident population of over 3000. These larger

villages and three main towns have generally been the locations for new

development, predominantly housing, over the past twenty years.

As well as areas at risk from flooding, the District has many other environmental

constraints. Over 60% of the District is covered by two protected landscape

designations, the High Weald AONB to the north (49%) and the South Downs

National Park to the south (11.2%). There are also numerous Sites of Special

Scientific Interest (SSSI), Sites of Nature Conservation Importance (SNCI) and Local

Nature Reserves (LNR’s) scattered throughout Mid Sussex and ancient woodland

covers 15.9% of the District’s area. The Ashdown Forest, although outside of the

District boundary, presents a further constraint as the 7km Zone of Influence

incorporates a large portion of the north and east of the District. The Forest is both a

Special Protection Area and Special Area of Conservation under EU regulations. An

assessment of primary and secondary constraints in the District has been undertaken

through the Capacity of Mid Sussex District to Accommodate Development Study

(June 2014).

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Figure 1 - Map of Mid Sussex District, identifying the main settlements and transport

corridors

The District does not have any areas of coastline, but does contain parts of

catchments of four main rivers; the River Ouse, the River Adur, the River Medway

and the River Mole.

1.3 The Mid Sussex Local Development Framework

Mid Sussex District Council are currently in the process of the producing a Local

Development Framework (LDF), which will replace the current Mid Sussex District

Council Local Plan, which was adopted in May 2004. The District Council has

produced the Mid Sussex District Plan 2014 - 2031 in preparation for submission to

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Government in 2015. The Environment Agency and other bodies have been

consulted on the contents of the District Plan throughout its preparation. The District

Plan seeks to achieve sustainable development in Mid Sussex in accordance with

the National Planning Policy Framework. Flood risk management can contribute

towards sustainable development by tackling climate change, using techniques that

enhance the natural environment and adopting strategies which don’t prevent future

generations from meeting their own needs and effectively managing their own flood

risk. Flood risk management can also enhance and protect the built, cultural heritage,

biodiversity, rural and natural environments by preventing loss and damage to

habitats and heritage assets, reducing pollution, protecting infrastructure and

transport links and contribute to income generation and economic growth.

Towns and Parishes within the District are producing their own Neighbourhood

Plans, 20 in total, setting the vision for their areas over the next 20 years and the

level of development they consider to be necessary and appropriate to help fund

improvements and meet local needs. Many of the Neighbourhood Plans also seek to

allocate sites for development to meet those needs.

The SFRA has been used help inform the content of the District Plan, particularly

with regards to the identification of strategic options for new development in the

district. The SFRA, which will be continually updated and will remain a ‘live

document’, will also be used in the determination of planning applications, including

proposals for windfall developments. Other Planning documents which the District

Council or Town and Parish Councils produce will utilise the SFRA including; the

Gypsy and Traveller Sites Development Plan Document (DPD), Neighbourhood

Plans and, if necessary, a Site Allocations DPD.

In undertaking the SFRA for Mid Sussex it is important to understand the

development pressures that are faced by the District over the coming years,

particularly in terms of new housing. Table 1 on the following page, which is taken

from Policy DP5 of the District Plan 2014 - 2031 sets out the District Council’s current

proposals for meeting the required housing figure of 11,050 new homes in the District

up until 2031.

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Table 1 - Housing Requirement for Mid Sussex 2014-2031

District Plan requirement 2014 - 2031 11,050

Completions 2014/15 (part year) 630

Total Housing Commitments 5,405

Strategic Development to the north and northwest of Burgess Hill (DP9)

3,500

Elsewhere in the District, as allocated through Neighbourhood Plans or other appropriate planning documents

1,515

In formulating the content of the above policy approach locations/options have been

considered and appraised in order to determine the most appropriate strategy for

meeting housing demand, including the ability of the District to meet some or all of

the unmet needs of neighbouring authorities and the impacts of doing so in

sustainability terms. Realistic options have been subject to the Sequential Test, as

defined in section 2.4 of this report, and Sustainability Appraisal. The Sequential

Test report for the District Plan can be viewed online at:

http://www.midsussex.gov.uk/8933.htm

http://www.midsussex.gov.uk/8933.htm

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2 Flood Risk

It is not technically, economically or environmentally feasible to prevent flooding

altogether, but the impacts of flooding can be avoided and reduced through

appropriate flood risk management.

2.1 What is flood risk?

Flood risk can be defined as a combination of the probability of a particular flood

event occurring and the impact that event would have. The probability of a particular

flood event is usually expressed as the probability of that flood event occurring in any

year, which in turn can be expressed as a percentage, Table 2 below demonstrates

this.

Table 2 – Flood risk probabilities

Annual flood probability as

percentage likelihood

Basis of expression

1% 1 in 100 chance of being equalled or

exceeded in any year

0.1% 1 in 1000 chance of being equalled or

exceeded in any year

2.2 Sources of flood risk

Flooding can occur from a variety of sources and can happen naturally, when specific

environmental factors or combinations of factors occur, or can be a result of human

intervention with natural processes, such as altering a river channel. Table 3, below,

provides an overview of the different flooding categories and their causes.

Table 3 - Categories and causes of flooding

Category Cause

Fluvial flooding When the amount of water in a watercourse exceeds the flow

capacity of the channel of that watercourse*. The excess water

spills onto the floodplain associated with that watercourse. Fluvial

flooding can also be caused where the channel of a watercourse

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becomes blocked; this usually occurs where culverts and bridges

become blocked by debris, or where constrictions in the river

channel cause flood water to backup.

Coastal and tidal

flooding

High tides, storm surges and wave action, often in combination.

Areas affected by such forms of flooding are generally low-lying

land by the sea and tidal estuaries.

Surface water

(pluvial) flooding

Surface water flooding results from excess overland flow and

ponding. This can be caused by intense short duration storms

when the water does not have time to soak into the ground or at

times when the ground is already saturated and water cannot

soak in at all. Within the built environment surface water will tend

to collect more easily because the water cannot infiltrate or drain

into the ground like it would do naturally. Poorly maintained

drainage systems and blockages can increase the likelihood of

surface water flooding and in urban areas this can include

pollutants (particularly sewage).

Groundwater

flooding

When water levels in the ground rise above surface elevations or

floods subsurface infrastructure. Low lying areas that are

underlain by aquifers are particularly susceptible to this form of

flooding, especially during the winter months.

Flooding from

sewers

The overloading or blockage of surface water sewers or

combined sewers**. Urban areas are particularly at risk from this

form of flooding when sewers can become overwhelmed by

heavy rainfall.

Flooding from

reservoirs,

canals and other

artificial sources

Flooding from reservoirs and canals can occur as a result of that

facility being overwhelmed with water or as a result of dam or

bank failure. Other causes of flooding under this category include

where current pumping systems fail or cease to be used resulting

in groundwater levels rising. This is particularly the case where

there are current or derelict minerals workings.

* the term watercourse includes rivers, streams, ditches and other natural passages

through which water flows.

** combined sewers are those sewers that contain both surface and waste water

(sewage).

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2.3 Implications of flood risk

Developments that are designed without regard to flood risk may endanger lives,

damage property, cause significant disruption to the economy and the wider

community, damage the environment, be difficult to insure and require additional

expense for remedial works. With this in mind, such developments would be

unsustainable and would therefore not be in accordance with paragraph 100 of the

National Planning Policy Framework (NPPF).

The implication of flooding to properties has been recently demonstrated locally and

nationally in flood events in early 2014, where properties were flooded across

Sussex, including in parts of Copthorne and Hassocks.

Appropriate flood risk management can achieve wide-ranging benefits including the

enhancement and protection of infrastructure, transport links, the built, rural and

natural environments as well as achieving economic growth and regeneration, habitat

enhancement and reduce pollution

2.4 The Strategic Flood Risk Assessment and Sequential Approach

The basis for all decision making in flood risk is to first understand the risk and then

identify responses to that risk so that it is effectively managed. The SFRA provides

information that should be supplemented, where necessary, with more detailed

information. Paragraph 100 of the NPPF provides the national policy background

concerning the need for an SFRA to be undertaken by Local Planning Authorities to

inform the preparation of Local Plans. Local Plans should also apply a “sequential,

risk-based approach to the location of development to avoid where possible flood risk

to people and property and manage any residual risk, taking account of the impacts

of climate change”.

The aim of the Sequential Test is to steer new development to the areas that are at

the lowest probability of flooding. For fluvial flood risk these areas are classed as

Flood Zone 1 but the sequential test should recognise the possible risk of flooding

from all sources. The fluvial flood zones are identified in Table 4 below (source: ID:

7, National Planning Practice Guidance (NPPG) – Table 1). As stated in paragraph

101 of the NPPF, “Development should not be allocated or permitted if there are

reasonably available sites appropriate for the proposed development in areas with a

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lower probability of flooding”. This applies in the determination of planning

applications and in the production of Local Plans and Neighbourhood Plans.

Table 4 - Flood Zones

Note: these flood zones refer to the probability of river and sea flooding ignoring the

presence of flood defences.

Flood Zone 1 (Low

probability) – comprises

of land having a less

than 1 in 1000 annual

probability of river or

sea flooding.

Appropriate uses

All uses of land are appropriate in

this zone.

Policy aims

Developers and local authorities should seek

opportunities to reduce the overall level of flood

risk in the area and beyond through the layout and

form of new development, and in particularly the

appropriate application of sustainable drainage

systems (i.e. reduce surface water run-off from

new development).

Flood Zone 2 (Medium


of land having between

a 1 in 100 and 1 in

1000 annual probability

of river flooding or

between a 1 in 200 and

1 in 1000 annual

probability of sea

flooding.

Appropriate uses

All uses are considered appropriate

but highly vulnerable uses of land

would require an exception test to

demonstrate that flood risk will be

managed satisfactorily. This

includes Police, Ambulance and

Fire Stations, emergency dispersal

points, basement dwellings,

caravans and mobile homes

intended for permanent residential

use.

Policy aims



risk in the area and beyond through the layout and

form of new development, and in particularly the

appropriate application of sustainable drainage

systems (i.e. reduce surface water run-off from

new development).

Flood Zone 3a (High


of land having a 1 in

100 or greater annual

probability of river

flooding or a 1 in 200 or

greater annual

probability of sea

flooding.

Appropriate uses

Water compatible uses (i.e. water

base recreation) and less

vulnerable uses of land (generally

buildings used for non-residential

uses, such as shops, offices and

agricultural buildings) as well as

essential infrastructure and more

vulnerable uses (including

residential) if an exception test can

show that flood risk to people and

property will be managed

satisfactorily.

Policy aims



risk in the area, through the layout and form of the

development (including application of SuDS), and

relocate existing development to land with a lower

probability of flooding. Opportunities should also

be sought to create space for flooding to occur by

restoring functional floodplains and flood flow

pathways and by identifying, allocating and

safeguarding open space for flood storage.

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Flood zone 3b (The

functional floodplain) –

land where water has to

flow or be stored in

times of flood. The

annual probability of

flooding would be 1 in

20 or greater.

Appropriate uses

Only water-compatible uses (i.e.

water based recreation) and

essential infrastructure that has to

be there.

Policy aims



risk in the area, through the layout and form of the

development (including application of SuDS), and

relocate existing development to land with a lower

probability of flooding.

It should be noted that flooding from surface water, groundwater, sewers, reservoirs,

canals and other artificial sources is not classified into these flood zones. The

information that the District Council has collected on flood risk from such sources will

be considered on a case-by-case basis to determine the severity of risk and whether

or not the cause of flooding could be resolved or overcome through new

development. In particular all major development of at least 10 dwellings or more and

major commercial development must ensure that sustainable drainage systems for

the management of run-off are put in place, unless demonstrated to be inappropriate.

Where it is not possible to allocate new development in areas of lowest risk due to a

lack of reasonable sites being available in such locations, then local authorities

should consider the allocation of development in sites that are classified as Flood

Zone 2. Consideration will need to be given to what uses are appropriate in such

areas (see table 4 above). Where suitable sites at lower risk of flooding are not

available then the Exception Test will be required in accordance with paragraph 102

of the NPPF. The Exception Test is a method to demonstrate and help ensure that

flood risk to people and property will be managed satisfactorily, while allowing

necessary development to go ahead. For the Exception Test to be passed:

it must be demonstrated that the development provides wider sustainability

benefits to the community that outweigh flood risk; and

a site-specific flood risk assessment must demonstrate that the development

will be safe for its lifetime taking account of the vulnerability of its users, without

increasing flood risk elsewhere, and, where possible, will reduce flood risk

overall.

Where no reasonable available sites can be found in Flood Zones 1 and 2, then only

in such an instance can the local authority consider development in Flood Zone 3.

Where the proposed use is considered not to be appropriate then the Exception Test,

as identified in the previous paragraph, will need to be applied.

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2.5 Neighbourhood Plans and Neighbourhood Development Orders

Parish and Town Councils involved in neighbourhood planning should (PPG ID: 7-

061-20140306):

seek to ensure neighbourhood plans and neighbourhood development/

community right to build orders are informed by an appropriate assessment of

flood risk;

ensure policies steer development to areas of lower flood risk as far as

possible;

ensure that any development in an area at risk of flooding would be safe, for

its lifetime taking account of climate change impacts;

be able to demonstrate how flood risk to and from the plan area/development

site(s) will be managed, so that flood risk will not be increased overall, and

that opportunities to reduce flood risk, for example, through the use of

sustainable drainage systems, are included in the plan/order.

2.6 Site specific Flood Risk Assessment

Site-specific flood risk assessment would generally be undertaken at the planning

application stage to ensure that flood risk is considered in the determination of

planning permission. Flood Risk Assessments should be undertaken under the

supervision of an experienced flood risk management specialist (who would normally

be expected to have achieved chartered status with a relevant professional body

such as the Institution of Civil Engineers (ICE) or the Chartered Institution of Water

and Environmental Management (CIWEM)).

Site-specific flood risk assessments are required for any site greater than 1 hectare

in Flood Zone 1; all proposals for new development (including minor development

and change of use) in Flood Zones 2 and 3; proposals in an area within Flood Zone 1

which has critical drainage problems (as notified to the local planning authority by the

Environment Agency); and where proposed development or a change of use to a

more vulnerable class (see Appendix E) may be subject to other sources of flooding.

Major development - developments of 10 dwellings or more; or equivalent non-

residential or mixed development (as set out in Article 2(1) of the Town and Country

Planning (Development Management Procedure) (England) Order 2010) are required

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to ensure that sustainable drainage systems for the management of run-off are put in

place, unless demonstrated to be inappropriate.

Where developers seek to provide new development, similar principles need to be

applied as the ones set out for development at a strategic level. These principles

concern locating development in the areas of lowest flood risk and only developing in

areas of higher risk (i.e. Flood Zones 2 and 3) where no reasonable available sites

can be found in lower risk areas. In such instances the Exception Test may need to

be applied.

The objectives of a site-specific flood risk assessment are to establish:

• whether a proposed development is likely to be affected by current or future

flooding from any source;

• whether it will increase flood risk elsewhere;

• whether the measures proposed to deal with these effects and risks are

appropriate;

• the evidence for the local planning authority to apply (if necessary, see below)

the Sequential Test, and;

• whether the development will be safe and pass the Exception Test, if

applicable.

Developers and applicants need to consider flood risk to and from the development

site and where sites are affected by above ground water features (these may be

outside of the site itself - see Appendix A, section (b)), site-specific flood risk

assessments should also demonstrate that they will not be at risk of flooding from

these.

Within the site, the most vulnerable development must be located in areas of lowest

flood risk unless there are overriding reasons to prefer a different location.

Development must be appropriately flood resilient and resistant, including safe

access and escape routes where required, it must be demonstrated that any residual

risk can be safely managed, including by emergency planning, and proposals must

give priority to the use of sustainable drainage systems (SuDS). Sustainable

drainage systems should always be provided as part of major development proposals

unless demonstrated to be inappropriate. South East England authorities have

prepared guidance for sustainable drainage titled ‘Water.People.Places. A guide for

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master planning sustainable drainage into developments’1 that should be used at the

earliest stages of the planning and design process for all types of residential,

commercial and industrial development. In order to determine the level of flood risk

for an individual site it is the responsibility of the developer to prepare a Flood Risk

Assessment (FRA). Applicants for planning permission (or prior approval in the case

of certain permitted development rights) should also refer to the Environment

Agency’s guidance2 on flood risk when preparing a site-specific flood risk

assessment.

The FRA should include:

a location plan showing geographical features, street names and identifies the

catchment, watercourses or other bodies of water in the vicinity;

a plan of the site showing the: (i) existing site; (ii) development proposals; and

(iii) any structures which may influence the local flood flow overland and/or in

any watercourses present on site;

site levels – both existing and proposed;

information about the surface water disposal measures already in place and

their state of maintenance;

an assessment of the volume of surface water run-off likely to be generated

from the proposed development;

allowance in design for how the increased frequency and intensity of rainfall

that is predicted as a result of climate change will affect the proposal;

information about other potential sources of flooding, if any, that may affect

the site e.g. streams, surface water run-off, sewers, groundwater, reservoirs,

canals and other artificial sources or any combination of these; including

details on how these sources of flooding will be managed safely within the

development proposal;

proposals for surface water management that aim to not increase, and where

practicable reduce the rate of runoff from the site;

confirmation as to whether Environment Agency consent is needed for any

aspect of the work, and whether this has been applied for or not;

information about how any residual risks are to be managed over the lifetime

of the development.

1 www.midsussex.gov.uk/media/Water_People_Places.pdf

2 https://www.gov.uk/flood-risk-assessment-for-planning-applications

http://www.midsussex.gov.uk/media/Water_People_Places.pdf

https://www.gov.uk/flood-risk-assessment-for-planning-applications

19

For individual developments on sites allocated in development plans (which are

accompanied by a Sequential Test) applicants need not apply the Sequential Test.

Applications for minor works and changes of use (except for any proposal involving a

change of use to a caravan, camping or chalet site, or to a mobile home or park

home site) should not be subject to the Sequential or Exception Tests but should still

meet the requirements for site-specific flood risk assessments. This is in accordance

with paragraphs 103 and 104 of the NPPF and the FRA should build upon the

findings of the SFRA.

2.7 The impact of climate change on flood risk

The water environment is particularly vulnerable to the effects of climate change.

Climate change hazards include brief events, such as severe storms, and longer

term trends. It is accepted that one impact will be more frequent, short duration, but

high intensity rainfall events as well as more frequent long-duration rainfall events.

As identified in Table 3, causes of flooding can include physical, biological and

human processes. Interactions between these multiple processes, which are already

complex, are changing over time as a consequence of climate change and it is

therefore necessary to adopt a proactive flood risk strategy to mitigate and adapt to

the predicted impacts of climate change and allow for uncertainties. Preventive action

brings clear economic, environmental and social benefits by anticipating potential

impacts and minimising threats to human health, ecosystems, the economy and

infrastructure.

This SFRA uses climate change allowances set out in PPS25. PPS25 recommended

that a 10% increase in peak river flow for the period up until 2025 is given for

considering the impact of climate change on fluvial flood risk. For the period after

2025 and until 2115 a 20% increase in peak river flows is given. The Environment

Agency issued guidance to support the NPPF in September 20133 that stated that

these remained the recommended climate change allowances for use in flood risk

assessments, plans and determining planning applications.

The baseline river flow is that used for the Environment Agency’s Flood Map. The

Agency’s flood zones can provide a rough indication of areas where climate change

3 Climate change allowances for planners, Environment Agency (2013)

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/296964/LIT_8496_5306da.pdf

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may have the most lateral effect (i.e. extent of the flood risk area), by comparing

Zones 2 and 3 to see how rare, more extreme flows affect the floodplain extents.

However increased flood levels will also be associated with more severe events and

following consultation with the Agency the SFRA has identified the potential impacts

of more severe flood events resulting in a 20% increase in peak river flows. In terms

of what these impacts would be the following has been agreed between the District

Council (including the Land Drainage Engineer) and the Environment Agency:

With an allowance for climate change, areas covered by Flood Zone 2

become areas of Flood Zone 3a.

With an allowance for climate change, areas covered by Flood Zone 3a

become areas of Flood Zone 3b (functional floodplain).

A 20 metre horizontal buffer strip is to be applied to the outer edge of the

existing areas of Flood Zone 2. As an allowance for climate change, this 20

metre buffer strip will be considered as an area of Flood Zone 2.

A 20 metre horizontal buffer strip is to be applied to the outer edge of areas

that have been identified as having historically flooded (this includes flooding

from overland flow as well as groundwater flooding).

With climate change predicted to bring about an increase in rainfall intensity, there is

the likelihood that there will be increases in surface water run-off and therefore

drainage schemes will need to be designed taking climate change into account. The

Environment Agency recommends a 5% increase in rainfall intensities by 2025, a

10% increase between 2025 and 2055, a 20% increase between 2055 and 2085 and

a 30% between 2085 and 2115.

There are many methods available to reduce the impact of runoff from development

throughout Mid Sussex and the ‘Water.People.Places.’ document provides a guide. A

case study showing the potential impact of climate change has been developed

below for flow attenuation in the form of ponds.

For example, an attenuation pond intercepting a catchment area of 4 hectares (runoff

rate of 0.8) designed to attenuate the flow for a 1 in 25 year return period storm of 1

hour duration from an average peak runoff rate of 350 l/s down to a Greenfield runoff

rate of 20 l/s, would need to have a capacity of 1,200 m3. A 20% increase in rainfall

(predicted between 2055 and 2085) would require the 1,200 m3 capacity of the pond

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to be increased by at least 20% to 1,440 m3 to be able to maintain the same level of

protection against flooding.

This example illustrates how important it will be for climate change allowances to be

built into the design of drainage systems throughout the district. In most instances

the Council will seek such drainage systems to be designed with a 30% increased

allowance for the impact of climate change.

22

3 Methodology and findings of the SFRA

3.1 Introduction

The approach to producing this SFRA has been guided by national policy and

guidance, West Sussex Local Flood Risk Management Strategy (2013 – 2018) and

advice obtained from the Environment Agency, which has included the Environment

Agency guidance document entitled “Strategic Flood Risk Assessments - Guidance

to support the National Planning Policy Framework”. This guidance note explains

that there are two levels of assessment to reflect the likely risk of flooding and

development pressures in the local planning authority area.

All Local Plans should be supported by a Level 1 SFRA that informs plan preparation

and enables the Local Planning Authority undertake the Sequential Test. In areas like

Mid Sussex where land outside flood risk areas can appropriately accommodate all

necessary development (i.e. where flooding isn’t a major issue or where development

pressures are low) this may be the only SFRA carried out. The sequential test for the

Mid Sussex District Plan 2014 – 2031 concludes that the proposals in the Plan direct

development of vulnerable uses to areas of low flood risk.

The key outputs of a Level 1 SFRA are:

Maps showing the local planning authority area, main rivers, ordinary

watercourses and flood zones, including the functional floodplain if

appropriate, as well as all previously allocated development sites (or sites to

be considered in the future);

An assessment of the implications of climate change for flood risk at allocated

development sites over an appropriate time period;

Areas at risk from other sources of flooding, such as surface water or

reservoirs;

Flood risk management measures, including location and standard of

infrastructure and the coverage of flood warning systems;

Locations where additional development may significantly increase flood risk

elsewhere through the impact on existing sources of flooding, or by the

generation of increased surface water run-off;

Recommendations about the identification of critical drainage areas and the

potential need for surface water management plans;

23

Advice on the preparation of flood risk assessments for allocated and other

development sites; and

Advice on the likely suitability of sustainable drainage techniques for

managing surface water runoff at key development sites

The Level 2 SFRA should be undertaken in areas where land outside flood risk areas

cannot appropriately accommodate all the necessary development. A Level 2 SFRA

should build on the source information comprised within a Level 1 Assessment and

also contain:

An appraisal of the current condition of flood defence infrastructure and of

likely future flood management policy with regard to its maintenance and

upgrade;

An appraisal of the probability and consequences of overtopping or failure of

flood risk management infrastructure, including an appropriate allowance for

climate change;

Definition and mapping of the functional floodplain in locations where this is

required;

Maps showing the distribution of flood risk across all flood zones from all

sources of flooding taking climate change into account;

Advice on appropriate policies for sites that could satisfy the first part of the

Exception Test (sustainability benefits to the community that outweigh flood

risk), and on the requirements that would be necessary for a site-specific

flood risk assessment in support of a planning application in order to pass the

second part of the Exception Test;

Advice on the preparation of site-specific flood risk assessments for sites of

varying risk across the flood zones, including information about the use of

sustainable drainage techniques; and

Meaningful recommendations to inform policy, development management and

technical issues.

It was not necessary to undertake a Level 2 Assessment at this time although some

elements of the Level 2 Assessment have been addressed where appropriate to do

so.

24

3.2 Coarse Assessment

The first stage of the SFRA was to undertake the ‘Coarse Assessment’. This

assessment entailed obtaining sufficient information on flood risk in the district to

enable the Sequential Test to be undertaken for the Core Strategy and any other

LDD’s that the District Council will be preparing. Information that was gathered

included the following:

The Local Planning Authority boundary

Location of main rivers

Location of all other watercourses

Locations of flood defences

Areas with flood warnings

Land that is classified as Flood Zone 2 or 3 (fluvial and tidal)

Land that has been subject to flooding from overland flow, groundwater and

sewers.

Land at risk of flooding from reservoirs, canals and other artificial sources.

Areas with flood management strategies.

Areas of groundwater vulnerability.

Areas of surface water flood risk.

Locations and details of historical flood incidents.

Geology within the district.

Topographical data within the district.

Environmental problems and/or strategies that are sensitive to flood

management activities.

Information from the District Council’s Emergency Planning function.

Reports in the local press (Mid Sussex Times and East Grinstead Courier) of

flooding incidents and events.

The findings and methodology for collecting this information/data is detailed in

Appendix A – Coarse Assessment data sources. This includes the sources for the

information/data, difficulties in collecting it and any uncertainties/gaps in knowledge.

All information/data has been plotted on the District Council’s GIS using the most up

to date version of Ordnance Survey’s (OS) Landline Basemap as a guide while

digitising. This is the same basemap used when viewing the SFRA layers on the

25

District Council’s GIS. Where individual properties have been highlighted, OS

AddressPoint was used in order to determine the location.

3.3 Other steps taken to obtain information on flood risk

The District Council has sought to make the SFRA as comprehensive as possible but

in some cases it has not been possible to acquire all the information that would

otherwise have been included. Steps were taken to obtain the following data:

Requests for updated flood incident data and modelling from Southern Water

and Thames Water were made but no additional information was received.

Several Parish and Town Councils did not respond to the District Council’s

requests for information on historic flood incidents.

However, as the SFRA is a live document any subsequent information or data that

becomes available will be added.

3.4 Have all hazards been sufficiently defined?

The District Council consider that they have identified all flood risk hazards as far as

practically possible. It is accepted that as the SFRA is a ‘living document’ there will

be further flood risk hazards identified over time, which will be added to the

information/data that has already been identified in this SFRA and the mapping that

accompanies this SFRA will be kept up-to-date. Any subsequent flood risk hazards

will therefore be taken into consideration in undertaking the Sequential Test for future

Local Development Documents.

3.5 Overview of flood risk in the district

The following table summarises aspects of this data and thereby provides an

overview of the district’s flood risk characteristics.

Table 5 – Overview of Mid Sussex flood risk characteristics

No Question Area (km2) % of Area

1 Size of District 334.029 N/A

2 Area in Zone 3 (High flood risk) 7.78 2.233 % of Total Area

3 Area in Zone 2 (Moderate flood risk) 9.72 2.91 % of Total Area

4 Existing development in Zone 3 0.215 2.763 % of Zone 3

5 Existing development in Zone 2 0.356 3.768 % of Zone 2

6 Area of Zone 3 that is defended 0.011 0.141 % of Zone 3

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7 Total Developed Area 32.306 9.672 % of Total Area

8 Required new development* 3.683 1.103 % of Total Area

9 Likely Development in Zones 3 and 2 0 0.000 % of Zones 3 and 2

10 Area affected by drainage problems 1.108 0.332 % of Total Area

11 Area affected by groundwater flooding 0.164 0.049 % of Total Area

12 Area affected by overland flows 0.427 0.128 % of Total Area

13 Area affected by surface water flood

risk (1 in 30 year)

7.25 2.17 % of Total Area


risk (1 in 100 year)

12.22 3.658 % of Total Area


risk (1 in 1000 year)

33.502 10.03 % of Total Area

* Takes the required housing delivery of approximately 11,050 new homes over the period

until 2031 and assumes this housing will be built at an average of 30 dwellings per hectare. It

should be recognised that a considerable amount of this development will be on previously

developed land. In addition, no allowance is given for new employment developments or any

other uses.

27

4 Outcomes

4.1 Implications for the District Plan 2014 - 2031

This SFRA has been prepared with the production of the District Plan in mind and is

therefore considered to be fit for purpose to inform the content of this document. Its

findings are also a relevant consideration in the production of other Development

Plan Documents and Neighbourhood Plans.

The information detailed in the previous section and Appendix A – Coarse

Assessment data sources has enabled the District Council to undertake the

Sequential Test for the District Plan 2014 - 2031. This Sequential Test is detailed in a

separate report but concludes that the proposals in the District Plan do not involve

the development of vulnerable uses in Flood Zones 2 and 3 and there is no

requirement for an Exception Test to be undertaken.

The information gathered in this SFRA, when read alongside the ‘Water. People.

Places.’ sustainable drainage guide, will be used to ensure that site specific flood risk

assessments for development allocations within the District Plan identify the most

appropriate form of SuDS (i.e. utilising the geological information gathered for the

SFRA and the planning and design guidance within Water. People. Places.) as well

as applying a more detailed Sequential Test for the location of different development

types within the development areas. In addition, the Gatwick Sub Region Water

Cycle Study makes a number of policy recommendations relating to the management

of surface water (see section (q) of Appendix A for details).

The SFRA has demonstrated that due to the nature of Mid Sussex, in terms of it

being at the source of the four river catchments that cover the district, the level of risk

of fluvial flooding is comparatively low when compared to neighbouring authorities.

However, what has been identified is the need for careful planning to ensure against

increased risks of flooding downstream (i.e. on the watercourses that pass into

neighbouring authorities). An example of this is on the tributaries of the River Mole

where existing flooding problems in Crawley could be exacerbated by inappropriate

drainage schemes for development within this catchment and within Mid Sussex

District. Similarly, where opportunities arise for the storage of water within these

areas this can help reduce the risk of flooding to downstream areas. Land at Clay’s

Lake and Worth Farm identified for flood alleviation schemes should be safeguarded

28

from development for this reason. Neighbourhood Plans may address locally specific

flood risk issues and where sites are allocated for housing and employment uses a

separate Sequential Test shall be undertaken for each Neighbourhood Plan.

4.2 Advice on the preparation of flood risk assessments for allocated development sites

Advice on the preparation of flood risk assessments for allocated development sites

is defined as a key output of a Level 1 SFRA within the National Planning Practice

Guidance. Similar principles apply for all development and further advice on site-

specific flood risk assessments can found in Section 2.6.

The strategic allocation to the east of Burgess Hill at Kings Way contains no areas of

current or future fluvial flood risk as defined within this SFRA but the development will

still need to take into account the movement of water, its interaction with

development proposals and integrate sustainable drainage solutions into the master

planning for the development. Some areas of surface water flood risk are indicated

based on modelling, these are near to the existing watercourses (ditches and

channels) running through the lowest parts of the site and using the sequential test

these areas should be avoided. Existing watercourse channels, ponds and flow

routes should be preserved and improved. The site is indicated as being at a low

level of groundwater flood risk potential due to clay geology.

The strategic allocation to the north and north-west of Burgess Hill (District Plan

policy DP9) contains areas identified as being at current and future (see Section 4.3)

risk within Flood Zones 2 and 3. Development in these areas should be avoided

using the sequential test. Modelling indicates areas of surface water flood risk; these

are near to the existing watercourses running through the lowest parts of the site.

West Sussex County Council have identified historic records of isolated surface

water flooding nearby around Bridge Hall Farm that should be investigated further as

part of any Flood Risk Assessment.

Proposals will need to consider flood risk management and the potential for the

development to increase the risk of flooding downstream. Masterplanning of the

development should be undertaken in order to direct vulnerable types of

development away from areas of current and future flood risk including the

watercourses running through the site (tributaries of the River Adur) and climate

29

change buffer zone. Consideration of flood risk issues must include areas at future

risk from climate change as required by Policy 4 of the River Adur Catchment Flood

Management Plan (December 2009) and identified in mapping for the SFRA (see

sections 2.7 and 4.3 for further details).

Both sites have been subject to a high-level assessment in the Gatwick Sub Region

Water Cycle Study and are considered to have low potential for infiltration, however

site specific infiltration tests should be undertaken to determine if local variations in

soil conditions will be suitable for infiltration SuDS.

4.3 Assessment of the implications of climate change for flood risk at allocated development sites

It is necessary to assess the potential impact of climate change on fluvial flood risk

for strategic allocations within the District Plan and this is a key output of a Level 1

SFRA. The approach applied for this SFRA is as recommended by the Environment

Agency and detailed within section 2.7 of this document.

As previously mentioned the strategic allocation to the east of Burgess Hill at Kings

Way is wholly located within Flood Zone 1. The site and its relationship with areas of

current and future fluvial flood risk are shown at Figure 2.

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Figure 2 - Climate change implications at DP8 Land to the east of Burgess Hill at Kings

Way

The strategic allocation to the north and north-west of Burgess Hill (District Plan

policy DP9) is a 218 hectare site with some existing areas within Flood Zone 2 and 3

particularly around the watercourses in the north-west of the site. The map at Figure

31

3 also shows that there are some areas of future fluvial flood risk as a result of

climate change to the south and east of the allocated area.

Development proposals will need to take areas of current and future flood risk into

account, take a sequential approach to flood risk and ensure that risks can be

managed through suitable adaptation measures in accordance with guidance in

Appendix C – Flood Risk Assessment and Sustainable Drainage Systems guidance

for specific areas within the District and ‘Water. People. Places’4. Proposals must

also ensure development is safe throughout its lifetime and must not increase the risk

of flooding elsewhere. Due to the geology of the Burgess Hill area, poor permeability

is likely to limit the effectiveness of SuDS that promote infiltration; however some

infiltration may still be achieved. SuDS should, therefore, primarily be designed to

provide the required attenuation and treatment above ground or near the surface.

There are a range of SuDS that could be appropriate in these circumstances (if

designed correctly) and possible options are identified within the ‘Water. People.

Places.’ guide. It is likely a number of these methods could be utilised across the site.

Other approaches such as the retention and expansion of woodland areas (see the

Woodland Trust’s “Stemming the Flow” guidance5) could be used to increase

resilience to future flood risk and support other policy objectives such as the delivery

of Burgess Hill Green Circle routes.

4 http://www.eastsussex.gov.uk/NR/rdonlyres/C849823F-F843-470A-A6E6-

44D6F2310691/0/SE7sudsmasterplanning_low_res_reduced.pdf

5 http://www.woodlandtrust.org.uk/mediafile/100188521/lga-flood-report.pdf

http://www.eastsussex.gov.uk/NR/rdonlyres/C849823F-F843-470A-A6E6-44D6F2310691/0/SE7sudsmasterplanning_low_res_reduced.pdf

http://www.eastsussex.gov.uk/NR/rdonlyres/C849823F-F843-470A-A6E6-44D6F2310691/0/SE7sudsmasterplanning_low_res_reduced.pdf

http://www.woodlandtrust.org.uk/mediafile/100188521/lga-flood-report.pdf

32

Figure 3 - Climate change implications at DP9 Land to the north and north-west of

Burgess Hill

4.4 Advice on sustainable drainage techniques for managing surface water run-off at development sites

Sustainable drainage systems are designed to control surface water runoff close to

where it falls and mimic natural drainage as closely as possible. They provide

opportunities to:

Reduce the causes and impacts of flooding;

Remove pollutants from urban runoff at source;

Combine water management with green space with benefits for amenity,

recreation and wildlife.

The aim should be to discharge surface run off as high up the following hierarchy of

drainage options as reasonably practicable:

into the ground (infiltration);

to a surface water body;

to a surface water sewer, highway drain, or other drainage system;

to a combined sewer.

33

Surface water flood mapping (see section (n) of Appendix A for further details)

identifies the countryside areas around Hickstead, Staplefield, Twineham and

Twineham Green as being at high risk of flooding from this source.

New development should only be considered appropriate in areas at risk of flooding if

priority has been given to the use of sustainable drainage systems. Following the

Ministerial Statement on sustainable drainage systems on 18th December 2014, now

reflected in Planning Practice Guidance, all developments of 10 dwellings or more

and other major developments are required to ensure that sustainable drainage

systems are put in place for the management of run-off, unless demonstrated to be

inappropriate. However, well-designed SuDS should cost no more, and usually less,

than conventional piped drainage systems and given the lower maintenance costs,

pollution removal and amenity benefits of most SuDS it is advised that all

developments should seek to implement sustainable drainage solutions.

Early consideration of sustainable drainage systems can ensure cost-effective

delivery, make the most of the potential benefits SuDS can offer and consequently

maximise the desirability and value of a development. Development sites should

ensure SuDS are interconnected and provide a treatment train through which water

is slowed down or stored and slowly released. This passes the runoff through a

variety of features offering several stages of treatment which results in improved

water quality for the water returning to the river networks. Infiltration techniques are

generally the preferred SuDS method because they provide source control close to

the location of run-off generation, however, there are areas within the district where

infiltration may not be appropriate such as where the impermeable nature of ground

conditions (for example, areas underlain by Weald Clay) minimise their effectiveness.

Treatment and management close to source can also avoid the need for significant

excavations and major engineer works. The emerging District Plan 2014 – 2031

Policy DP41 Flood Risk and Drainage incorporates the preferred hierarchy for

managing surface water drainage in developments.

Drainage elements require periodic inspection and maintenance, especially features

designed to intercept silt or oil. Wherever possible these functions should be

provided within an above ground SuDS system, where access can readily be

achieved and the need for maintenance is easily apparent. The Ministerial Statement

requires local planning authorities to consult the Lead Local Flood Authority (West

Sussex County Council) on planning applications for surface water management,

34

ensure that the proposed minimum standards of operation are appropriate and that

there are clear arrangements in place for ongoing maintenance over the lifetime of

the development. The sustainable drainage system/s should be designed to ensure

that maintenance and operation requirements are economically proportionate.

SuDS should reflect the character of the local area; the predominantly rural nature of

the District suggests that SuDS with soft edges and natural features will usually be

most appropriate. The Capacity of Mid Sussex to Accommodate Development Study

identifies primary and secondary constraints across the district that may affect the

suitability of some SuDS, in particular environmental designations and the high

quality landscape that affect large areas of Mid Sussex. Where possible, and

particularly where additional benefits could be achieved by doing so, SuDS should be

designed to include plants6, trees and other vegetation to store water and provide

filtration as well as improving and complementing the ecology and amenity of the

area.

Appendix C provides specific guidance taking into account location and geology in

certain areas of the district that should be used to inform site-specific Flood Risk

Assessments and the design of SuDS.

4.5 Recommendations about the identification of critical drainage areas and the potential need for surface water management plans

The Environment Agency has not notified us of any critical drainage problems within

the district.

Three potential locations for the development of Surface Water Management Plans

(SWMPs) have been identified. The first is identified in the River Adur Catchment

Flood Management Plan and the Gatwick Sub-Region Water Cycle Study which

recommend the development of a SWMP for Burgess Hill (see sections (l) and (q) of

Appendix A for more information). The River Ouse Catchment Flood Management

Plan and Gatwick Sub-Region Water Cycle Study also identify network capacity at

Haywards Heath as a potential constraint and recommend that a SWMP is

considered to determine where drainage improvements can be delivered. West

6 Planting should not impair the drainage function

35

Sussex County Council plan to undertake a Surface Water Management Plan for

Hassocks in 2015/16 with a Grant-in-Aid for £425,000 between 2016 and 2019.

The approach and level of analysis within a SWMP should be proportionate to the

risk and complexity of the area concerned. Technical Guidance7 is available from the

Department for Environment, Food and Rural Affairs but is primarily intended to be

used for areas of high flood risk.

4.6 Areas at risk from other sources of flooding

Generally Mid Sussex is an area of low flood risk however there are areas affected

by specific issues and careful management is necessary to ensure flood risk is not

increased now or in the future. Analysis undertaken for the West Sussex Local Flood

Risk Management Strategy identifies ‘wet spots’ where a limited number of properties

are considered to be at risk. These are Burgess Hill, East Grinstead, Haywards

Heath/Lindfield and Sayers Common (mostly surface water flood risk) and Copthorne

and Hassocks (both surface water and fluvial flood risk). There are no areas

identified at risk from coastal and tidal flooding. Locations where mapping identifies

large areas considered to be at high risk of surface water flooding are rural locations

south of Staplefield and around Hickstead, Twineham and Twineham Green.

Reservoir flooding is extremely unlikely to happen but the Environment Agency has

produced mapping8 to indicate the worst case scenario in the event that a reservoir

was to fail. This mapping indicates that a limited area around existing watercourses

to the south of Ardingly reservoir (countryside areas to the north of Haywards

Heath/Lindfield/Cuckfield) and in the immediate vicinity around Weir Wood reservoir

(countryside areas to the south of East Grinstead/Ashurst Wood) could be affected,

but it is unlikely that any actual flood would be this large.

The majority of the district is considered to have medium potential for groundwater

flooding however a small area within the South Downs National Park is considered to

have high potential for flooding from this source.

7 www.gov.uk/government/publications/surface-water-management-plan-technical-guidance

8 http://watermaps.environment-agency.gov.uk/wiyby/wiyby.aspx?topic=reservoir

http://www.gov.uk/government/publications/surface-water-management-plan-technical-guidance

http://watermaps.environment-agency.gov.uk/wiyby/wiyby.aspx?topic=reservoir

36

4.7 Recommendations for Development Management purposes

SFRA mapping can be used as a constraint layer when analysing planning

applications, deciding which organisations need to be consulted and when a Flood

Risk Assessment (FRA) needs to be undertaken by an applicant. There are a

number of specific flood risk considerations identified that may need to be addressed:

mapping identifies locations within the district that currently experience

problems with drainage and will enable the case officer to consult with the

District Council’s Drainage Team and/or the Lead Local Flood Authority

and/or the Environment Agency on planning applications that will be relevant

to them;

the coarse assessment (Appendix A) identifies watercourses and

recommends that a twenty metre precautionary buffer zone is applied.

Applications for development within these buffer zones will need to

demonstrate that they will not be at risk from flooding from these

watercourses;

all areas covered by Flood Zone 3 are to be treated as areas of Flood Zone

3b unless evidence can be provided to demonstrate otherwise. In particular

the Environment Agency have advised that land that would naturally flood

with an annual probability of 1 in 20 (5%) or greater in any year should be

identified as functional floodplain;

it will be essential that the recommendations and guidance concerning

climate change impact, as summarised in section 2.7 of this report, are

followed with regards to designing drainage schemes and determining what

areas will likely be at risk from flooding, and what the level of risk will be, as a

result of climate change. The SFRA has highlighted the importance of climate

change allowances being built into the design of drainage systems throughout

the district. Proposals should ensure drainage systems are designed with a

30% increased allowance to protect against the impacts of climate change.

Further information about undertaking a site-specific flood risk assessment (FRA)

can be found in section 2.6 of this document. It is expected that any such

assessment would draw upon the information and data gathered for this SFRA.

It will be essential for applicants to carefully consider the land take implications of

sustainable drainage systems at the earliest opportunity. The ‘Water. People. Places’

37

guide details the range of SuDS available for use and the type of conditions that best

suit their implementation. It is recommended that this guidance is closely followed

when considering the implementation of SuDS. Appendix C – Flood Risk

Assessment and Sustainable Drainage Systems guidance for specific areas within

the District of this SFRA provides further information on particular issues that need to

be considered in specific areas of the district.

The District Council’s Planning Policy Division can supply SFRA mapping information

to applicants on request ([email protected]).

4.8 Recommendations to inform future policy

The SFRA has identified a number of areas that would improve flood risk

management in Mid Sussex and reduce the risk of flooding both in the district and in

neighbouring areas. This document will inform future policy decisions, including for

Neighbourhood Plans.

Specifically, additional storage of water on the floodplain, particularly in rural parts of

the Low Weald and High Weald AONB, has the potential to significantly reduce flood

risk to downstream areas by restoring rivers and floodplains to a naturally functioning

state and contributing to biodiversity improvements. Preservation and restoration of

natural landscape features (such as forests, floodplains and wetlands) are important

components of rural land management, providing floodwater retention and

groundwater recharge. This approach has already reduced flood risk in Crawley and

Horley as part of the Upper Mole Flood Alleviation Scheme. Woodland schemes

could increase water retention alongside other benefits such as improved wildlife and

habitat connectivity and the expansion of green infrastructure networks. If an area is

intended to flood then this should be safeguarded from development and identified as

functional floodplain even though it might not flood very often.

Actions that will maintain and improve river flows in urban areas are encouraged. In

particular, the River Ouse Catchment Flood Management Plan identifies the long-

term protection and re-creation of the Scrase Bridge Stream and West Common

Stream at Lindfield.

mailto:[email protected]

38

4.9 Mapping

Mapping produced to accompany this SFRA shows administrative boundaries, main

rivers, ordinary watercourses, flood zones (including the functional floodplain where

appropriate) and allocated development sites, as well as other information collected

as part of the coarse assessment (see Section 3.2 and Appendix A – Coarse

Assessment data sources). In order to ensure up-to-date and relevant mapping this

will be updated with new data releases and flood incidents as and when these

become available. Therefore, a distinct set of maps will not be available to

accompany the SFRA, except those contained within this document, but mapping will

be freely available from the District Council on request.

The Flood Map for Planning (from Rivers and Sea)9 and Risk of Flooding from

Surface Water map10 produced by the Environment Agency are also available as

online resources.

4.10 Flood risk management measures

A variety of flood risk management measures have been undertaken by landowners,

developers, local authorities and other groups across the district and it would not be

possible to provide a comprehensive list but some of the measures implemented by

the District Council are shown in Appendix B - Flood improvement/alleviation

measures undertaken by the District Council. Two flood alleviation schemes in north-

west Mid Sussex have been implemented as part of the River Mole Flood Risk

Strategy (see section (l) of Appendix A for details).

Worth Parish Council have received Operation Watershed funding and are working

with both West Sussex County Council and Mid Sussex District Council to deliver a

new trash screen at The Green, along with earth movements to create a water

detention area in the vicinity of the new screen. These improvements aim to reduce

flood risk in the village. The Parish Council have also applied for a grant to set up an

emergency flood plan with local volunteers.

9 http://apps.environment-agency.gov.uk/wiyby/37837.aspx

10 http://watermaps.environment-agency.gov.uk/wiyby/wiyby.aspx?topic=ufmfsw

http://apps.environment-agency.gov.uk/wiyby/37837.aspx

http://watermaps.environment-agency.gov.uk/wiyby/wiyby.aspx?topic=ufmfsw

39

Environment Agency flood warning areas11 cover properties in the immediate vicinity

of tributaries of the River Adur (particularly around Albourne, Bolney, Hurstpierpoint &

Sayers Common and Twineham parishes), small parts of the countryside around the

River Ouse (in Ardingly and Lindfield Rural parishes) and around Scrase Stream in

Lindfield. Mapping showing these areas is available as an online resource. The

Council’s Emergency Plan and Severe Weather Plan, as well as the West Sussex

Multi-Agency Flood Plan could be applied during a flood incident. The West Sussex

Multi-Agency Flood Plan sets out when a response should be triggered.

4.11 Future use of the SFRA

As mentioned throughout this report this SFRA is a ‘live document’ in that information

and data can be constantly added to that which has already been collated and

mapped. This information and data can then be used in applying the Sequential Test

and determining policy requirements in subsequent DPD’s and SPD’s that the District

Council produce.

11



40

5 Limitations, restrictions and statements of accuracy

In some instances, historical flooding information and/or data that has been supplied

to the District Council has not identified the cause of the flooding or when it took

place. In these cases it has been decided to either leave these gaps in information

or, through consultation with the District Council’s Drainage Team, determine what

the missing information/data is likely to be. It is accepted in the latter case there will

be some instances where there may be slight inaccuracies in the information

reported.

Whilst the District Council are confident that the data supplied by external sources is

accurate, we are not responsible for any inaccuracies involved, whether relate to

background details (date of flooding, type of flooding, etc.) or with the area/address

identified. The Environment Agency highlighted that indicative floodplain information

is out of date and information on historic flood events is of variable quality and

accuracy and so this data should be read with those considerations in mind.

Section 3.3 of this report identifies information and data sources that the District

Council attempted to obtain for the purposes of this SFRA but were not forthcoming.

In light of this it is considered that the SFRA is not entirely comprehensive. However,

every reasonable attempt has been and will continue to be made to obtain

information on flood risk affecting the district.

In the production of this SFRA there has been a reliance on external sources to keep

the District Council informed with regards to data/information updates. To ensure

that the SFRA remains an up-to-date ‘living document’ there will be a need to ensure

that these arrangements are maintained and the District Council updates GIS data as

required.

The District Council will attempt to stay well informed and plot new flood events when

they occur in order to maintain an up-to-date dataset. To ensure consistency, every

attempt will be made to follow the same principles in capturing new data as those

used when plotting data captured in writing this version of the SFRA.

Appendix A – Coarse Assessment data sources

a) Local Planning Authority boundary

Included on the District Council’s GIS is an accurate boundary of the district, as well

as boundaries for all parishes and district wards. This information can be overlaid

onto the OS basemap, together with the all the other SFRA layers.

b) Location of main rivers and all other watercourses

Prior to undertaking this SFRA the District Council had already obtained the GIS

layer identifying the main river centrelines that are within the district. The District

Council contacted the Environment Agency with a view to obtaining the most up-to-

date information on fluvial flood risk, including the main river centrelines for its 2008

SFRA. This information has been incorporated into the SFRA mapping on the

District Council’s GIS. Updates to this information (supplied to the District Council by

the Environment Agency) have and will be incorporated/amended within the SFRA

mapping.

In terms of other watercourses the OS basemap layer on the District Council’s GIS

has been used to identify all other above ground water features within the district by

isolating the “0059 – Standard Water Features” OS feature style on the Landline

dataset, and formatting it in blue. This includes streams, ditches and drainage

channels large enough to be picked up by the surveying techniques. This

information has been incorporated into the SFRA mapping on the District Council’s

GIS. A twenty-metre precautionary buffer zone should be applied for planning

purposes (as agreed in consultation with the Environment Agency). Any applications

for development within these zones will have to demonstrate that they will not be at

risk from flooding from these watercourses. By having such a buffer will ensure that

any areas that may be at risk from flooding, from these minor watercourses, as a

result of climate change will be identified in site-specific Flood Risk Assessments.

c) Locations of flood defences

Significant river flood defences are identified in the West Sussex Strategic Flood Risk

Assessment (2010) but none of these are currently located within Mid Sussex

District. The Environment Agency have previously supplied the District Council with

42

mapping that identifies the location of flood defences built since 2001 to protect

against river floods with a 1% (1 in 100) probability of happening each year, together

with some, but not all, older defences and defences which protect against smaller

floods. These defences will be incorporated into the SFRA mapping as and when

new information is available from the Environment Agency. Also identified on this

mapping are the areas that benefit from the presence of these defences (i.e. the

areas that would flood under a 1 in 100 year flood event if the defences were not

there). This information has also been supplied by the Environment Agency and they

are looking to continue adding to this information.

Mid Sussex District Council is a Land Drainage Authority and has permissive powers

in respect of ordinary watercourses. The District Council hold records of flood

defence measures that have been implemented to help reduce localised flooding

problems. These measures have generally been balancing ponds/attenuation ponds

but include culverts and trash screens. Many of these features are included within

the SFRA mapping, however there are some flood improvement/alleviation measures

that have not been mapped and these are identified in Appendix B - Flood

improvement/alleviation measures undertaken by the District Council. Further

information on areas affected by these flood defence measures can be obtained from

the District Council’s Drainage Team.

d) Areas with flood warnings

Environment Agency Flood Warning and Alert Area mapping12 is available online and

shows the areas where alerts and warnings are available to residents, the current

flood status of these areas and provides a record of previously issued flood

warnings/alerts. This includes properties in the immediate vicinity of tributaries of the

River Adur (particularly around Albourne, Bolney, Hurstpierpoint & Sayers Common

and Twineham parishes), the River Ouse (in countryside around Ardingly and

Lindfield Rural parish) and around Scrase Stream in Lindfield. GIS layers that cover

those areas have been incorporated into the SFRA mapping on the District Council’s

GIS.

12



43

e) Coastal and tidal flooding

Mid Sussex does not border any stretches of coastline and is therefore not at risk

from coastal flooding. The watercourses within Mid Sussex are all influenced by the

tide up until a certain distance inland. The stretches of these watercourses where

they are tidally influenced are outside of the district (i.e. the River Adur is influenced

by the tide up until Chates Weir and Mockbridge, both locations are near to Henfield

and lie within Horsham district). As a result of these findings there are no areas

identified at risk from coastal and tidal flooding on the SFRA mapping on the District

Council’s GIS.

f) Fluvial flooding

In terms of information/data on fluvial flooding the Environment Agency has been the

main provider. The Southern (Solent and South Downs) Area office is the main point

of contact for Mid Sussex and they were able to supply information and data for the

whole Area which covers the Adur, Ouse, Mole and Medway catchments.

Information and data that they have provided and which has been included on the

SFRA mapping on the District Council’s GIS is as follows:

Main River locations within Mid Sussex boundary.

Indicative floodplains – Fluvial, within Mid Sussex boundary.

Historic flood events within Mid Sussex boundary.

Flood Map for Planning (Zones 2 and 3) within Mid Sussex boundary.

Flood Map for Surface Water (1 in 30yr, 1 in 100yr and 1 in 1000yr).

In supplying this data the Agency highlighted any limitations, restrictions or

statements of accuracy concerning the datasets. They stated that the indicative

floodplain information is out of date and no longer used by the Agency and that the

information on historic flood events is of variable quality and accuracy. An allowance

for climate change was not included within the Flood Zone mapping supplied by the

Agency. Section 2.7 of this report details how such an allowance has been built into

this mapping.

With regards to areas covered by Flood Zone 3, the PPG sub-divides this zone into

Zone 3a (High Probability) and 3b (The Functional Floodplain). It is stated that Level

44

2 SFRA’s should identify the areas covered by Flood Zone 3b. The identification of

functional floodplain should take account of local circumstances and not be defined

solely on rigid probability parameters. However, land which would naturally flood with

an annual probability of 1 in 20 (5%) or greater in any year, or is designed to flood

(such as a flood attenuation scheme) in an extreme (0.1% annual probability) flood,

should provide a starting point for consideration to identify the functional floodplain.

There are also areas of high surface water flood risk (1 in 30 annual probability) that

should be considered as the functional floodplain, particularly in locations where

there is nowhere else for the water to drain to. Due to this information not being

available for most of the district, and the District Council not having the resource to

map these areas it has been decided to take the precautionary approach and to

assume that all areas covered by Flood Zone 3 are to be treated as areas of Flood

Zone 3b (the exception being the area of the district within the Upper Mole

catchment, see below) unless up to date evidence demonstrates otherwise.

Areas which would naturally flood, but which are prevented from doing so by existing

defences, infrastructure and/or solid buildings, will not normally be identified as

functional floodplain. If an area is intended to flood, e.g. an upstream flood storage

area designed to protect communities further downstream, then this should be

safeguarded from development and identified as functional floodplain, even though it

might not flood very often.

In terms of fluvial flooding information and data for the Upper Mole catchment, the

District Council received data sets from the Agency. Included within these data sets

were the following:

Flood extents from a detailed modelling study carried out on the Upper River

Mole catchment in 2000. Flood extents produced include the 1in 5, 1 in 20

and 1 in 100 year flood events. The Agency stated that the 1 in 20 year flood

event outline can be used in the SFRA to identify areas of functional

floodplain (Flood Zone 3b). An allowance for climate change was not

included in this study. Section 2.7 of this report details how such an

allowance for climate change has been built into this Flood Zone mapping.

Main River Map showing extents of Main River and Non-Main River in the

north-west area of Mid Sussex District.

The above two data sets have been included on the SFRA mapping.

45

In addition to the above mentioned datasets the District Council already had the

following datasets, concerning fluvial flooding, included on GIS:

Areas benefiting from defences (supplied by the Agency)

Location of flood defences (supplied by the Agency)

Location of flood storage areas (supplied by the Agency)

Historic flood map (supplied by the Agency). This mapping shows the

combined extents of known flooding from rivers and groundwater.

Main river centrelines (supplied by the Agency).

This mapping will be updated when the Agency provide any new information.

In addition to the information provided by the Environment Agency, the District

Council also collected information on fluvial flooding from other individuals and

organisations. This information was predominantly received from either the District

Council’s own Drainage Team and Town and Parish Councils (the methodology and

reasoning for collecting this information from these organisations is identified in

section (i) of this appendix). Historical fluvial flood events that were identified

generally resulted from flooding of ordinary watercourses; the Environment Agency’s

Flood Zone Mapping primarily shows flooding from Main Rivers, not ordinary

watercourses with a catchment of less than 3km. Where individuals/organisations

identified such events they were asked to provide information on the date(s) of the

flooding incidents, the cause, the area affected and whether any flood alleviation

measures have been put in place. This information has been included within the

SFRA mapping.

g) Surface water flooding incidents (also see section (n))

A large amount of information and data has been supplied by the Land Drainage

section, particularly records on properties that had experienced flooding as a result of

major flood events in December 1993, October/November 2000 and December 2013

– February 2014. Where a flooding incident had been recorded and its cause had

been identified as overland flow, the property/area affected was identified and

captured on the SFRA GIS layer. When only a road name was supplied, the full

extent of the road was captured using the road centreline. If any alleviation works

had been undertaken to help prevent this flooding from happening in the future, or to

46

reduce the risk, this was documented on the mapping in the background data held for

each incident.

In addition to information received from the Land Drainage section, the District

Council also contacted all the Town and Parish Council’s within the District, the

Emergency Services and the County Council Highways and Transport Department to

obtain records of historical flooding they may hold.

Risk of Flooding from Surface Water mapping identifies approximate areas at risk of

flooding (see section (n) of this appendix). The County Council Highways and

Transport Department also provided a list of 14 locations within the district that

experience persistent flooding problems. These locations were all on public

highways and the cause of the flooding was identified as the inability of rainfall to

enter the drainage system and thereby cause overland flow. In some cases a whole

highway was identified but the District Council has attempted to identify specific

location(s) of where flooding occurred wherever possible.

The District Council contacted the West Sussex Fire and Rescue Service (WSF&RS)

to obtain any records they may hold of flooding to properties. It was considered that

they might hold such records as in instances where they would have been contacted

to pump out properties or rescue inhabitants of buildings that are flooding.

Unfortunately only limited information was available but the District Council has

included all of these incidents within the SFRA mapping. WSF&RS were unable to

identify the sources of these flooding incidents. As these incidents were all a

considerable distance from any watercourses the District Council considered that the

flooding would not have been caused by over-topping of watercourses (fluvial

flooding). Also, as all the flooding incidents were during the summer months it was

presumed that the flooding would not have been caused from raised groundwater

levels. Therefore, the District Council stated that the source of these flooding

incidents was either from overland flow or flooding from sewers.

h) Groundwater flooding incidents (also see section (m))

As identified in table 3 of this report, groundwater flooding is seen as a characteristic

of low-lying areas that are underlain by aquifers. Large parts of the district are

underlain by aquifers, particularly the chalk aquifers in the southern part of the district

and the sandstone aquifers in the High Weald area of the district. Parts of the district

47

where these aquifers lie are also low-lying. This includes part of the southern area of

the district, particularly Hassocks, Hurstpierpoint and Sayers Common.

The District Council sought to obtain data on where flooding from high groundwater

levels had occurred from Town and Parish Council’s, the Environment Agency and

the District Council’s own Drainage Team. Only a limited number of historical flood

records, where the cause had been documented as groundwater flooding, were

obtained. It is possible that some of the records of flooding, where the cause was

recorded as overland flow, may have been in part caused by water levels in the

ground rising above surface elevations however it has not been possible to

determine if this is the case with any certainty. Areas where the cause of flooding is

known to be groundwater have been included within the SFRA mapping.

The West Sussex Strategic Flood Risk Assessment (2010), produced by Capita

Symonds on behalf of West Sussex County Council, identified a single groundwater

flooding event at Cuckfield.

i) Flooding from sewers

Data collected from Southern Water and Thames Water

Southern Water provides wastewater treatment services across the majority of the

district, with Thames Water providing the same service for the north-western part of

the district (Copthorne and Pease Pottage area). The District Council originally

contacted these two utility companies on the 6th October 2006 requesting information

on any flooding incidents they hold records for and then again in March 2015

requesting updated information. The type of information requested included dates

and locations of flooding incidents, the frequency of these incidents and the causes.

The District Council first met with officers from Southern Water on the 26th October

2006 to discuss the SFRA process and how Southern Water could assist with this.

Further to this meeting, Southern Water provided historic data on flooding problems

up to January 2007 as well as providing predictions of flood volumes using available

models (these models would be based on the piped network and would predict where

water would flood from the system in a 1 in 100 year storm event). Following

consultation with the Environment Agency it was decided that using the historical

data would be the best form of data to use for the SFRA (the Agency considered that

the time and costs associated with detailed modelling would not add sufficient benefit

48

to the SFRA). Following a request for updated flood incident data, Southern Water

provided up to date on 22nd May 2015. The Council attempted but was not able to

obtain similar records from Thames Water.

The data provided showed flooding incidents that had occurred within the district over

the last 18 years. Included within this information was the date of the flooding

incident, the locality (street, settlement, post code, grid reference), the type of sewer

where flooding occurred from and whether the flooding affected properties internally,

within their curtilage or on the highway (the information provided was in the form of

an excel spreadsheet).

This information should be used to identify an issue that would need resolving before

further development could proceed in that location, rather than identifying a location

where further development would not be allowed. Examples of how this information

could be used include; identifying policy requirements for the allocation of sites;

setting a Development Management policy for certain areas of the district; and

making the District Council, applicants and developers aware of these issues at the

start of the planning application process. However, current data does not identify any

issues at the locations within the strategic allocations of the District Plan 2014 –

2031. Whilst a number of incidents have been identified across the district there are

currently no specific locations where a bespoke policy should be applied but planning

applications at locations affected by or adjacent to recorded historical incidents

should demonstrate whether issues have been or can be resolved.

In identifying this information on the SFRA mapping it has been found that each

record only identifies one particular property, due to the grid reference provided, but

in some cases more than one property has been affected. On the District Council’s

GIS there is a Public Sewer layer identifying the location of sewers under the

jurisdiction of Southern Water Services Ltd. This GIS layer specifies whether the

sewer is a surface water sewer, a foul sewer or a combined sewer. Where Southern

Water have identified a historical flooding incident on a certain street the data on the

District Council’s SFRA GIS layer has been captured using the line of the sewer on

the Public Sewer layer as a guide. For flooding incidents where the sewer has been

identified as ‘foul/combined’ the current operational ‘foul’ sewer along this street has

been used. For flooding incidents from ‘surface water’ sewers the ‘surface water’

sewer has been used. In certain instances a historical sewer flooding record has

been identified on a street that is shown not to have a sewer running along it. In

49

such instances the data on the SFRA layer has been captured using the road

centreline for the full extent of the street.

It is recognised that some of these historical flooding problems from sewers have

been resolved, however where this has happened it has not been identified by

Southern Water. Therefore, taking the precautionary approach, all records have

been identified on the SFRA mapping with the caveat that further consultation will be

required with Southern Water on any development proposals that will impact on the

sewers/areas identified, on the SFRA mapping, to establish the extent of the issue

and what needs to be done to resolve it, or whether it has already been resolved.

Data collected from town and parish councils and district councillors

All town and parish councils within the district were written to between the 7th

November and 10th December 2014. These organisations/individuals were

requested to supply information on historical flooding incidents in their local area,

preferably identifying the source/cause of flooding, when the incidents occurred and if

any alleviation scheme had been put in place to resolve the flooding problem. They

were also asked to review any existing historical flood incident records for their area,

including those identified during a previous consultation by the District Council in

2006, and identify where issues had been resolved. In writing to the town and parish

councils a map was enclosed to enable areas/properties that have flooded in the past

to be identified. Maps were sent out at a variety of scales, anything between 1:8000

and 1:26000 depending on the size of the Parish. Therefore, when being captured

onto the District Council’s SFRA GIS layer, areas highlighted as having historical

flood events are interpretations of the data supplied, although every effort has been

made to ensure they are of high accuracy. The reason for contacting the town and

parish councils was that it was considered these individuals/organisations would

have a good knowledge of their local area, which would include information on past

flood events.

In total there are 24 town and parish councils within Mid Sussex, all of which were

written to. In response, the District Council received information on previously

unidentified historical flood incidents in Ardingly, Balcombe, Hassocks, Lindfield

Rural and West Hoathly.

50

Not all town and parish councils responded to the request for flood event/risk

information and therefore the SFRA is perhaps not as extensive as it could be.

However, a lack of response may indicate a lack of historic flooding incidents in those

towns and parishes. It was possible to supplement this information with historic flood

incident data received by West Sussex County Council in January and February

2011 as part of their Local Drainage Asset Survey and other flood incident data

shared by local groups. This data identified incidents in Bolney, Burgess Hill,

Cuckfield, Fulking, Horsted Keynes, Poynings, Turners Hill, Twineham and West

Hoathly. In addition, consultation with the District Council’s Drainage Team, who

keep records of historic flood events, should ensure that most historical flooding

problems have been documented in this SFRA.

Much of the information received from town and parish council’s was also obtained

from the District Council’s Drainage Team. In some instances the town and parish

council’s had not identified the cause of past flooding incidents. In many instances

the information/data gathered by the District Council from other individuals and

organisations enabled a cause to be established. Where the cause was identified as

blocked or inadequate sewers the areas/properties affected, including the actual

sewers, were included within the SFRA mapping.

j) Flooding from reservoirs, canals and other artificial sources

There are no canals within the district and the one major reservoir is at Ardingly. The

reservoir is owned and run by South East Water. Reservoirs in the UK have an

extremely good safety record with no incidents resulting in the loss of life since 1925

and are carefully maintained to prevent flooding. Reservoir flooding is therefore

extremely unlikely to happen but the Environment Agency has produced mapping13

to indicate the worst case scenario in the event that a reservoir was to fail. This

mapping indicates that a limited area around existing watercourses to the south of

Ardingly reservoir (countryside areas to the north of Haywards

Heath/Lindfield/Cuckfield) and in the immediate vicinity around Weir Wood reservoir

(countryside areas to the south of East Grinstead/Ashurst Wood) could be affected,

but it is unlikely that any actual flood would be this large.

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51

A generic off-site plan for reservoirs has been prepared by Sussex Resilience Forum

to ensure a swift and effective response to any reservoir emergency involving

reservoirs for which specific off-site plans have not been established. It sets out the

co-ordination and control arrangements at each level of response across all

agencies.

The District Council are aware of only one significant scheme where water has been

pumped from one area to another within the district. This was at the Keymer Brick &

Tile Works site in Burgess Hill; however the site has currently ceased operations and

is allocated for residential development. Typically rainwater gravitates to the lowest

point of the pit area and to prevent flooding of the site the water was pumped into the

River Adur via a series of settlement ponds. This discharge into the Adur was

licensed by the Environment Agency. The site is underlain by Weald Clay which is

classified as a non-aquifer and is unable to transmit appreciable volumes of water

into the ground. This information has been included within the SFRA mapping and

will need to be considered if any development is to occur on this site.

k) Flooding from outside the District boundary and from inside out

Due to the nature of Mid Sussex, the district experiences only minor flood risk

problems from areas outside of its administrative boundary. This is due to the source

of the watercourses such as the Ouse, Adur, Medway and Upper Mole being within,

or just outside of the district. Hence, Mid Sussex does not experience downstream

problems from these watercourses as a result of activities in a neighbouring

authority. The downside of this is that what happens in Mid Sussex can impact on

flood risk in neighbouring authorities. This would generally occur where development

in Mid Sussex would result in an increase in surface water being discharged into a

watercourse, which would be likely to impact negatively on flood risk downstream,

particularly in urban areas such as Crawley. As identified in section (q) of this

appendix, schemes are being implemented within Mid Sussex to reduce the risk of

flooding in Crawley, Horley and at Gatwick Airport as part of the Upper Mole Flood

Alleviation Scheme. It is important that proposed new development in the district as

well as the Council’s Local Development Framework fully consider cross-boundary

flood risk implications.

52

l) Areas with flood management strategies

West Sussex Local Flood Risk Management Strategy

West Sussex County Council as Lead Local Flood Authority are required to set out

how it will deliver local flood risk management under the Flood and Water

Management Act. West Sussex County Council have prepared a Local Flood Risk

Management Strategy covering the county to meet their duties as a Lead Local Flood

Authority. The analysis undertaken in this strategy identifies 53 ‘wet spots’ in West

Sussex with six identified in Mid Sussex at the following locations:

Burgess Hill

Copthorne

East Grinstead

Hassocks

Haywards Heath/Lindfield

Sayers Common

The majority of properties identified are in areas of surface water flood risk, however

there a number of properties at risk of river flood risk in Copthorne and Hassocks.

The most significant clusters of properties are located in the urban centres,

particularly Haywards Heath and Burgess Hill. Purely surface water flooding is known

to exist in East Grinstead and on a smaller scale in Sayers Common. Properties are

largely at risk from surface water flooding in Hassocks. No wet spots within Mid

Sussex are identified as ‘priority’ wet spots by the County Council.

Wet Spot

Surface Water

Flood Risk

(no. of

properties)

River and Sea

Flood Risk

(no. of

properties)

Combined Flood

Risk (properties

within surface &

river/sea water

risk areas)

Total (no. of

properties)

Burgess Hill 2,500 1 0 2,501

Copthorne 75 130 75 280

East Grinstead 2,200 0 0 2,200

Hassocks 525 105 55 685

53

Haywards Heath &

Lindfield

2,200 0 100 2,300

Sayers Common 50 0 0 50

The Strategy identifies that Burgess Hill was previously known to have experienced

regular sewer flooding.

Catchments

The Environment Agency has defined catchments where inter-connected water

bodies converge to a single point and the resulting Catchment Flood Management

Plans are used to manage water issues in an integrated way across authorities. The

south of the District (up to the southern boundary of Haywards Heath) is covered by

the River Adur Catchment Flood Management Plan (CFMP) (December 2009). The

north of the district is split between three catchment areas; the villages of Copthorne,

Pease Pottage and the surrounding countryside fall within the Thames Region CFMP

area along with Crawley and Gatwick Airport. East Grinstead and the surrounding

villages fall within the Medway CFMP area and Haywards Heath and villages to the

north-east and west are within the Ouse CFMP (December 2009) area.

River Adur Catchment Flood Management Plan

The River Adur Catchment Flood Management Plan (December 2009) aims to

deliver sustainable long term flood risk management for the catchment area by

identifying flood risk management policies to assist decision making. The main

sources of flood risk in the Adur catchment area are from both localised river flooding

and surface water flooding, including flooding in urban areas due to under capacity

of, or blockages in, the drainage network. There have been several serious flood

events in the catchment area over the last century caused by surface water run-off

from the South Downs. The Plan is intended to inform local authority spatial planning

activities, as well as informing project and investment plans for the Environment

Agency, utility companies, transport planners, businesses and land

owners/managers whilst assisting the public.

The Burgess Hill and Hassocks area is identified within the CFMP as being an ‘area

of low, moderate or high flood risk where we are already managing the flood risk

effectively but where we may need to take further actions to keep pace with climate

54

change’ (CFMP Policy 4). The Plan also predicts that the number of properties in

Burgess Hill and Hassocks at risk will increase from 13 to 250 by 2100. The

proposed actions for Burgess Hill and Hassocks include strengthening development

management advice, increasing the use of SuDS through local development

framework policies, and developing a Surface Water Management Plan for Burgess

Hill.

Rural areas to the south and west of the Burgess Hill/Hassocks area (Upper Adur

and South Downs - East) are identified as ‘areas of low to moderate flood risk where

we will take action with others to store water or manage run-off in locations that

provide overall flood risk reduction or environmental benefits’. The proposed actions

for the Upper Adur include investigating opportunities where additional storage of

water on the floodplain could reduce flood risk to downstream areas, restore rivers

and floodplains to a naturally functioning state, and contribute to meeting biodiversity

action plan targets.

River Ouse Catchment Flood Management Plan

The Plan predicts that the number of properties in Haywards Heath at risk in a 1%

annual probability river flood is 27 and this figure is predicted to rise to 50 by 2100.

The Haywards Heath area is identified as an area of ‘low, moderate or high flood risk

where we are already managing the flood risk effectively but where we may need to

take further actions to keep pace with climate change’. The CFMP identifies the

potential for localised flooding from the Scrase Bridge Stream and West Common

Stream as a result of surface water overwhelming urban drainage systems. It is

recommended that policies are developed to work towards long-term protection and

re-creation of the Scrase Bridge Stream and West Common Stream and that a

Surface Water Management Plan is developed for Haywards Heath.

Twenty four properties in the High Weald and Middle Ouse area are considered to be

at risk of impacts in a 1% annual probability river flood event but this is not expected

to increase by 2100. This includes areas outside Mid Sussex in Wealden district. The

area is identified as an area of ‘low to moderate flood risk where we will take action

with others to store water or manage run-off in locations that provide overall flood risk

reduction or environmental benefits’. An increase in flood storage in this area could

reduce flood risk to properties in Uckfield and Lewes into the future. The use of agri-

55

environment and woodland schemes to increase water retention in the catchment

should be considered.

A small area around Scaynes Hill is located within the Ouse Low Weald area which is

an area of low flood risk that is not expected to increase in the future due to climate

change.

River Medway Catchment Flood Management Plan

The area of Mid Sussex within the River Medway catchment area is entirely within

the Upper Catchment sub-area and is identified as an area ‘where the risks are

currently appropriately managed and where the risk of flooding is not expected to

increase significantly in the future’. Development within the town of East Grinstead

should follow national policy, the Strategic Flood Risk Assessment and the Local

Flood Risk Management Strategy produced by West Sussex County Council in order

to manage flood risk and the speed of surface water run-off.

River Thames Catchment Flood Management Plan

The area of Mid Sussex within the River Thames catchment area is entirely within the

Upper Mole sub-area and is identified as an area of ‘low to moderate flood risk where

actions to store water or manage run-off in locations that provide overall flood risk

reduction or environmental benefits are recommended’. The approach to flood risk

management in these places uses the natural protection already provided by the river

channel and the open spaces in the floodplain. The proposed actions include

ensuring that recommendations in Strategic Flood Risk Assessments and Local

Development Framework policies create the potential to reduce flood risk through

adaptation of places at risk, managing run-off and retaining open spaces in the

floodplain.

The main aim of the River Mole Flood Risk Strategy is to reduce the level of flood risk

on parts of the Upper Mole catchment, which could have knock-on effects

downstream. Records show that severe flooding incidents have taken place within

the Upper Mole catchment in 1947, 1968, 1980, 1990, 1993, 1994, 2000, 2002, 2013

and 2014. The flood events in 2000 caused a significant number of properties to

suffer from flood damage at Fetcham, Dorking, Maidenbower, Furnace Green and

Ifield Green. Although all of these areas are outside of Mid Sussex, some of the

56

measures proposed to reduce the flood risk in these areas will impact upon the

district.

The Environment Agency, in partnership with the Upper Mole Strategy Working

Group14, identified two flood alleviation schemes within Mid Sussex, which will be

used to store and attenuate flow into the downstream watercourses, thereby reducing

the risk of flooding during heavy rainfall/storm events. It is essential that such areas

are safeguarded from development so that the implementation of these flood

alleviation measures is not compromised. The scheme at Worth Farm has now been

implemented and the scheme at Clay’s Lake is at an advanced stage at the time of

writing. It is anticipated that these works will be completed ahead of the adoption of

the Mid Sussex District Plan 2014 – 2031. This land should be safeguarded from

development in accordance with District Plan Policy DP41 Flood Risk and Drainage.

m) Areas of Groundwater Vulnerability

To assist in identifying areas that are at risk from groundwater flooding and to also

assist in the preparation of SuDS guidance for particular areas/sites in the district

information regarding areas that have vulnerability to groundwater flooding have

been included on the SFRA mapping. This mapping has, therefore, identified the

locations and extents of the major and minor aquifers within the district. The data for

this GIS layer was received from the Environment Agency prior to this SFRA being

undertaken. Information on spring lines was also obtained from the Council’s

Drainage Team and this has enabled areas at risk of groundwater flooding to be

identified.

The majority of the district is considered to have medium potential for groundwater

flooding. A small area of the district, the southernmost part within the National Park,

is considered to have high potential and the settlements of Burgess Hill, Hassocks,

Hurstpierpoint, Albourne and Sayers Common, as well as countryside areas to the

west are considered to be in an area of low potential for groundwater flooding.

14

This group consists of Local Authorities within the strategy area, which includes Mid Sussex

District Council, as well as other key stakeholders such as BAA Gatwick and Thames Water.

57

n) Areas of surface water flood risk

West Sussex Local Flood Risk Management Strategy identifies surface water

flooding as causing “the most regular impact to communities across West Sussex”.

The PPG (ID: 7-013-20140306) states that SFRAs should identify areas at risk from

surface water flooding taking account of the surface water flood risk map and other

available evidence such as local flood risk management strategies.

The Environment Agency’s Risk of Flooding from Surface Water mapping15 shows

the approximate areas at risk of flooding based on ground levels and drainage.

Locations at high risk of surface water flooding (annual probability greater than 1 in

30) are particularly notable south of Staplefield and around Hickstead, Twineham and

Twineham Green.

Catchment Flood Management Plans (see section (l) of this appendix for details)

have identified the main urban areas of Burgess Hill, East Grinstead and Haywards

Heath as having the potential for surface water flood risk. Historical flood events have

also occurred in the Low Weald caused by surface water run-off from the South

Downs, particularly at times of high groundwater levels.

o) Geology within the district

In order to identify the most appropriate SuDS strategy for different sites and areas

within Mid Sussex it is important to understand what the ground conditions are for

these different sites and areas. Ground conditions are likely to dictate whether or not

an infiltration drainage method, such as permeable paving or soakaways, would be

suitable or not. This information was obtained from the British Geological Society,

who provided GIS layers for the whole of the district, including areas of artificial

ground, bedrock classification, linear features, areas of mass movement and

superficial (surface) ground conditions.

15



58

p) Topographical data within the district.

To help understand how sites and areas currently drain it was important to gain

detailed topographical information, as far as possible, for the district. The

Environment Agency were able to supply LiDAR tiles, which provided contour data

for a number of the areas within the district, particularly in and around Haywards

Heath and Burgess Hill.

It is considered that this information will be of use to those preparing detailed site

specific FRA’s.

q) Environmental problems and/or strategies that are sensitive to flood

management activities

Gatwick Sub Region Water Cycle Study

The Gatwick Sub Region Water Cycle Study (January 2011) provides strategic level

advice on water infrastructure and environmental capacity in order to develop an

integrated approach to management of the water environment. Flood risk

management is a key part of this work and the assessment of environmental

constraints within the study finds that the assessed levels of development16 can be

accommodated within areas at low risk of fluvial flooding. The study does however

contain a number of relevant recommendations that should inform the preparation of

development plan documents. The study makes specific Local Plan policy

recommendations including:

a preferred hierarchy of managing surface water drainage from any

development through first infiltration measures, secondly attenuation and

discharge to watercourses, and if these cannot be met, through discharge to

surface water sewers only;

all developments to incorporate SuDS to control surface water run-off; and

a policy is adopted to ensure redeveloped brownfield sites disconnect any

surface water drainage from the foul network.

16

Due to the timing of the study the assessment reviewed the potential impact of growth

based on housing figures within the now revoked South East Plan.

59

Infiltration techniques are generally the preferred SuDS method because they

provide source control close to the location of run-off generation. These

recommendations have been incorporated into District Plan policy DP41 Flood Risk

and Drainage.

The Study also identifies that potential constraints to development exist relating to

the capacity of the sewerage network as well as the capacity of the wastewater

treatment works, especially in Burgess Hill and Haywards Heath. Sewerage

providers consider Surface Water Management Plans (SWMPs) a valuable tool in

alleviating network capacity issues, by addressing surface water management and

reducing storm overflows into the combined sewer system. The Study recommends

that SWMPs are considered for Burgess Hill and Haywards Heath to determine

where improvements in drainage can be delivered.

Catchment Flood Management Plans

Catchment Flood Management Plans (CFMPs) are a strategic planning tool through

which the Environment Agency will seek to work with other key decision-makers

within a river catchment to identify and agree policies for sustainable flood risk

management. As previously mentioned there are four catchments within Mid

Sussex; the Adur, Ouse, Medway and Mole.

A significant amount of information on flood risk is identified within these CFMP’s,

including within the Scoping Reports. They identify estimated depths of flooding

when a 1 in 100 year flood event takes place and also estimate costs for the flood

damage that would occur under such an event. Much of this information is of

relevance to the SFRA and will be used when applying the Sequential Test for the

allocation of land for development, as well as for the formulation of policies (further

information can be found in section (l) of this appendix).

Upper Mole Flood Alleviation Scheme

The Upper Mole Flood Alleviation Scheme is a series of schemes that, once

complete, will reduce the risk of flooding for at least 1,038 properties in the Crawley

and Horley areas as well as Gatwick Airport. Two of these schemes are located

within Mid Sussex District. Already constructed works include a new flood detention

60

reservoir adjacent to the M23 motorway at Worth Farm, and the construction of a

larger replacement dam at Clay’s Lake is currently underway. A Construction

Management Plan was submitted to Mid Sussex District Council in 2014 detailing the

scheduling of works at Clay’s Lake to provide the new flood defence by the end of

2015. Worth Farm lies to the east of the M23 near Junction 10A and close to the

motorway. This consists of a new embankment dam about 6.5 metres above the

surrounding ground, broadly parallel with the motorway. For most of the year, the

area can still be used for farming but when flows of water increase, the existing brook

will start to pond up behind the dam and will create a reservoir. This reservoir will

have a restricted outlet meaning that it will empty at a steady rate as water levels in

the brook fall. Clays Lake is an existing dam that is being upgraded. When water

flows are high the amount of water stored in the reservoir will increase before

returning to normal as water levels in the river fall. This allows the release of water at

a steady rate which should reduce the risk of flooding downstream in Crawley.

r) Information from the District Council’s Emergency Planning function

The District Council’s Emergency & Outdoor Service Manager confirmed that the

Council’s Emergency Plan and Severe Weather Plan could both be applied to a

flooding incident. The Council is also working with partners across West Sussex to

develop Part 2 Multi-Agency Flood Plans for areas at risk of flooding and these

complement the Sussex Resilience Forum’s adopted Part 1 Multi-Agency Flood Plan.

The West Sussex Multi-Agency Flood Plan sets out when a response should be

triggered and was activated and used during flooding in June 2012. If a response is

activated, adverse weather arrangements are supported by the Sussex Emergency

Response and Recovery Document and Multi-Agency Strategic Co-ordinating Group

Guidance. The multi-agency response will either be an Adverse Weather

Teleconference (chaired by the Environment Agency), an Adverse Weather Office

(chaired by the Police), or, to set up a Strategic Co-coordinating Group (chaired by

the Police).

With regard to rescue procedures the detailed plans for each urban centre contain

the processes involved to evacuate, and also include shelter arrangements.

Procedures and the response to flooding can vary depending on the type of flood

event, the area and the time of year. Membership of the recovery group will vary

depending on the event, but will usually include all risk management authorities of

61

which Mid Sussex District Council is one. A Recovery Co-ordinating Group led by

West Sussex County Council will manage the recovery process.

The District Council will not be impacted by any Shoreline Management Plans as the

District has no coastline. There are, however, likely to be other plans and strategies

that will influence the SFRA and when these documents are identified they will be

included within the SFRA.

s) Searching for reports in the local press (Mid Sussex Times and East

Grinstead Courier of flooding incidents and events

A number of flood incidents were identified:

Date Location Settlement Details

31st January

2004

Hanlye Lane Cuckfield West Sussex Fire and Rescue

Service had to pump water out

of the basement of a house

31st January

2004

Ardingly Road Lindfield Several houses flooded

1st February

2004

Delta House,

Bridge Road

Industrial Estate

Haywards

Heath

Two foot of floodwater due to

blocked culverts

8th November

2008

Keymer Road Hassocks Three shops flooded due to

blocked drains

4th August

2010

Elite Garage,

opposite Railway

Station

Haywards

Heath

Forecourt and workshop flooded

(surface water)

17th January

2014

Balcombe

Railway Tunnel

Balcombe Delays of 1hrs 40mins for train

services

17th January

2014

A23 northbound

(between A272

and M23) and

southbound

(between A264

and A272)

Carriageway flooded, A23

closed between those junctions

14th February

2014

Parklands Road Hassocks Garden flooding due to surge of

water in Herring Stream during

62

heavy rainfall. Culvert under

Downs View Road not large

enough to handle very heavy

rainfall.

28th July 2014 Collingwood

Road

East

Grinstead

One house and garage flooded

Information provided in the local press about these events is often incomplete or

limited and it is usually not known if the problems have been resolved. A

precautionary approach makes further consideration necessary (through a site-

specific flood risk assessment) for any development proposals that will impact on the

areas identified. It will be important to establish the likely cause and whether any

action needs to be taken to resolve the risk wherever possible. These incidents have

been included on the SFRA mapping.

Appendix B - Flood improvement/alleviation measures undertaken by the District Council

Dolphin Balancing Pond, Haywards Heath – Construction of a balancing pond.

Balancing Pond, Lincoln Wood, Haywards Heath – Raising of the banks of the

existing pond.

Penland Road, Haywards Heath – Construction of a balancing pond, clearing and

regrading stream and raising existing banks.

Concorde House, Balcombe Road, Haywards Heath – Clearing of rubbish from

banks of the stream.

Builders Centre, Bridge Road, Haywards Heath – Construction of a permanent

sandbag barrier.

Drummond Close, Haywards Heath – Bank construction works and landscaping.

By Sunte, Lindfield – Raising banks of the stream.

Meadow Lane, Burgess Hill – De-silting stream

Chanctonbury Road, Burgess Hill – Improvements to outlet

Herring Stream, Hassocks – Stream and bank clearance and de-silting of culvert.

Gleave Close Balancing Pond, East Grinstead – Construction of a balancing pond

and improvements to retain more water.

The Street, Bolney – Construction of large relief culvert.

Sydney Road, Haywards Heath – Construction of relief culvert.

Janes Lane, Burgess Hill – Improvements to culvert entrance and placement of

new screen.

Pyecombe – Improvements to drainage ditch.

Greenways, Haywards Heath – Construction of relief culvert.

Penland Road, Haywards Heath – Construction of relief culvert.

Oakhurst, Sayers Common – Construction of relief culvert.

Longhurst, Burgess Hill – Relining of foul sewer and improvement works.

Sandy Vale, Haywards Heath – Construction of relief culvert.

Norton House Car Park, Tower Close, East Grinstead – construction of a storm

water attenuation tank with restricted outlet.

Copthorne Common Road – installation of a new trash screen.

Hickmans Lane Recreation Ground – Construction of a balancing pond

For further information on these flood improvement/alleviation measures please

contact the District Council’s Drainage Team.

Appendix C – Flood Risk Assessment and Sustainable Drainage Systems guidance for specific areas within the District

This Appendix provides guidance on FRAs and SuDS that should be used when submitting/considering a planning application for development.

The document ‘Water. People. Places.’ should also be used as it provides excellent guidance for choosing SuDS. These guidance notes have

been prepared in partnership with the District Council’s Drainage Team. It is suggested that pre-application consultation is undertaken between

the developer and the District Council/Environment Agency/WSCC prior to submitting a planning application for development where flood risk

may be an issue.

Settlement/Area FRA guidance SuDS guidance

Pyecombe,

Poynings & Fulking

- the Downland

villages.

Surface water run-off from the Downs needs to be considered as do any

springs/spring lines that can be found in this locality.

In most instances in this locality, infiltration systems (permeable paving, soakaways, etc.) should be

able to be implemented effectively. Care needs to be taken to avoid pollution to the aquifers in the

area.

Hassocks (inc.

Keymer)

Flood Zones 2 and/or 3 (fluvial flood risk from the Herring Stream) would

need to be considered if applications are within these areas (a FRA would

need to accompany any application for development in such areas).

Proposals in and around Damian Way, Church Mead, Newlands Close,

Pattendens Nursery, Adastra Avenue, Queens Drive and The Quadrant

would need to consider historical fluvial flood events and the current risk of

flooding from all sources (areas are not covered by Flood Zones). A number

of minor drainage ditches/streams pass through and around the settlement -

the risk of flooding from these ditches/streams will need to be considered in

any FRA for a development proposal in their vicinity.

On the southern periphery of the settlement and the land immediately to the south, ground

conditions (mudstone) will mean that infiltration systems are unlikely to be suitable and therefore

attenuation systems, such as balancing ponds, should be considered as the preferred SuDS.

Permeable paving and other such systems can, however, be utilised so they act as source control

and storage prior to a restricted discharge to a watercourse. This also applies on land to the north of

the settlement, where the geology is generally Weald Clay with poorly drained soils. Within the

majority of the settlement itself, ground conditions are slightly more permeable (sandstone) and

should allow for infiltration systems or a combined infiltration/attenuation system. Land in this

location overlies a major aquifer and therefore high groundwater vulnerability will need to be

addressed in any SuDS design.

Hurstpierpoint and

Albourne

Due to the impermeable nature of the ground in this area surface water

drainage needs to be carefully considered. A number of minor drainage

ditches/streams are present on the northern, western and eastern sides of

Hurstpierpoint - the risk of flooding from these ditches/streams will need to

be considered in any FRA for a development proposal in their vicinity.

Springs/spring lines that are present on an east-west line through both

settlements will need to be considered.

For the majority of Albourne together with the southern half of Hurstpierpoint (and land to the south

of the village) the ground conditions mainly consist of sandy clays overlying Weald Clay. Minimal

soakage potential is present in this area (as can be demonstrated by the number of ponds in the

area) and therefore attenuation systems, such as balancing ponds, are going to be the most

appropriate form of SuDS in this area. Permeable paving and other such systems can, however, be

utilised so they act as source control and storage prior to a restricted discharge to a watercourse.

Land in this location overlies a major aquifer and therefore high groundwater vulnerability will need

65


to be addressed in any SuDS design. Much of the northern part of Hurstpierpoint and land to the

north of the village is underlain by Weald Clay and therefore the same issues concerning the use of

SuDS apply to this area as they do for the area to the south.

Sayers Common

and Hickstead area

(inc. Bolney

Grange)

Numerous records of historical flooding exist in and around Sayers Common

predominantly due to poor surface water drainage systems - this issue will

need to be carefully considered in any FRA in this locality. A significant area

of Flood Zone 2 and 3 (fluvial flood risk from the River Adur) runs through

the Hickstead area and to the immediate north of Bolney Grange - this will

need to be considered for any applications within this area (a FRA will need

to accompany any planning application for development in such areas).

The majority of this area is underlain by Weald Clay (the only exceptions are around the tributaries

of the River Adur where clay, silts, sands and gravels overlie the Weald Clay). The scope to utilise

infiltration systems will be minimal and therefore attenuation systems will be the most appropriate

form of SuDS. Evidence of the impermeable nature of the ground conditions in this area can be seen

by the numerous ponds in the locality. Permeable paving and other such systems can, however, be


Burgess Hill Tributaries of the River Adur lie immediately to the north of the built up area

boundary of the town, to the west and south west of the town and also

through the eastern part of the town. There are significant areas of fluvial

Flood Zone 2 and 3 associated with these watercourses - this will need to be

considered for any applications within this area (a FRA would need to

accompany any application for development in and adjacent to such areas,

which will need to consider the implications of climate change). The Worlds

End area of Burgess Hill (north east part of the town) suffers from localised

flooding problems resulting from overland flow and surcharging sewers

during periods of heavy rainfall. Any FRA in this locality will need to consider

these issues and demonstrate that this problem will not be exacerbated, and

where possible reduced or resolved. Further information on this issue can be

gained from the District Council's Drainage Team. Throughout the town

surface water drainage will need to be carefully considered for any

development work and as part of any FRA, if undertaken.

The majority of this area is underlain by Weald Clay (the only exceptions are around the tributaries

of the River Adur where clay, silts, sands and gravels overlie the Weald Clay). The scope to utilise


form of SuDS in and around the town. Permeable paving and other such systems can, however, be


66


Bolney The area around The Street, Bolney, has historically suffered from flooding

resulting from overland flow. Works have taken place to alleviate this

problem; however any further development in this locality will need to

consider whether the alleviation works are able to cater for any increase in

surface water flow. This should be demonstrated in any FRA for

development in this location.

The geology of the land in and around Bolney generally consists of sandstone and siltstone with

poor to moderately drained soils. The exception to this is an area of land that stretches westwards

from Bachelor's Field where the ground conditions consist of Weald Clay. Bearing in mind these

ground conditions, for large developments particularly (that will generate significant amounts of

surface water run-off) attenuation systems will be the most appropriate form of SuDS in this locality.

There may be scope to implement infiltration systems in combination with an attenuation system,

particularly for smaller developments. Land in this location overlies a minor aquifer and therefore

high groundwater vulnerability will need to be considered in any SuDS design

Ansty and

Cuckfield

No specific flood risk issues in and around Ansty. For Cuckfield, the area

around Broad Street (east of Warden Park School) has suffered from a

couple of minor floods as a result of overland flow from the school site.

Development in this locality should bear this in mind.

For Ansty the ground conditions consist of sandstone and siltstone with moderately drained soils on

gentle slopes. Combined infiltration and attenuation facilities are likely to be the most appropriate

form of SuDS in this locality. Land in this location overlies a minor aquifer and therefore high

groundwater vulnerability will need to be addressed in any SuDS design In Cuckfield the majority of

the area in and around the village is underlain by Upper Grinstead Clay, hence the scope to utilise

infiltration systems, such as soakaways, will be minimal.

Haywards Heath

(inc. Lindfield and

Scaynes Hill)

A number of watercourses pass through the northern part of Haywards

Heath and Lindfield, including the Scrase Stream and Northlands Wood

Stream (sections of both are classified as Main River). There are significant

areas of Flood Zone 2 and 3 associated with both of these watercourses.

Any application for development within these areas will need to be

accompanied by a FRA, which will need to address the issue of fluvial flood

risk (inc. considering the implications of climate change). Within Haywards

Heath there are a number of minor watercourses around Penland Road,

Burrell Road and Bridgers Mill that have areas of flood risk associated with

them and they have been known to overtop causing flooding of property and

roads. Any application for development in these areas will need to consider

this issue in a FRA. There are a number of other minor watercourses and

ditches in and around Haywards Heath and Lindfield so any proposed

development within their immediate vicinity should undertake a FRA to

ensure that the proposal will not be at an unacceptable risk of flooding. Any

development on the eastern side of the town (west of Cuckfield) will need to

consider the impact on the attenuation facilities/ flood alleviation measures

installed in this area to reduce flood risk downstream (see Appendix B -

The general geology in this area is sandstone and siltstone with moderately drained soils. With this

in mind infiltration systems are only likely to be appropriate to a certain extent, particularly on large-

scale developments. Hence it is recommended that combined infiltration and attenuation systems

will be the most appropriate form of SuDS in most instances. Such a system has been implemented

for the Bolnore Village development. Land in this location overlies a minor aquifer and therefore high

groundwater vulnerability will need to be addressed in any SuDS design.

67


Flood improvement/alleviation measures undertaken by the District Council).

For Scaynes Hill, overland flow (run off from the side of the hills) needs to be

considered for the area around the Millennium Village Centre.

Horsted Keynes No specific flood risk issues in and around Horsted Keynes that need to be

considered in addition to current FRA guidance.

The general geology in this area is sandstone and siltstone with moderately drained soils. With this

in mind infiltration systems are only likely to be appropriate to a certain extent, particularly on large-

scale developments. Hence it is recommended that combined infiltration and attenuation systems

will be the most appropriate form of SuDS in most instances. Land in this location overlies a minor

aquifer and therefore high groundwater vulnerability will need to be addressed in any SuDS design.


Balcombe An area of land either side of the Haywards Heath Road (particularly to the

north around the Old Rectory / Braky Wood) has experienced flooding in the

past as a result of an inadequate drainage system. Any proposed

development in this locality will need to demonstrate how this problem will

not be exacerbated and where possible rectified within the FRA. No other

specific flood risk issues in and around Balcombe that need to be considered

in addition to current FRA guidance.

Much of the area in and around Balcombe is underlain with Upper Grinstead Clay (although there

are areas of Ardingly Sandstone and Lower Tunbridge Wells Sand Formation) with poorly drained

soils. Minimal soakage potential is present in this area (as can be demonstrated by the number of

ponds and ditches in the area) and therefore attenuation systems, such as balancing ponds, are

going to be the most appropriate form of SuDS in this area. Permeable paving and other such

systems can, however, be utilised so they act as source control and storage prior to a restricted

discharge to a watercourse.

Ardingly No specific flood risk issues in and around Ardingly that need to be

considered in addition to current FRA guidance.

In the western part of the village the ground conditions consist of clay overlain with poorly drained

soils. With this in mind attenuation systems, such as balancing ponds, are going to be the most

appropriate form of SuDS in this area. In the remainder of the village and land to its east the geology

is mostly mudstone and sandstone and therefore the same guidance applies in this area with

regards to SuDS. Permeable paving and other such systems can, however, be utilised so they act

as source control and storage prior to a restricted discharge to a watercourse.

West Hoathly and

Sharpthorne

The area around The Hollow, which runs between West Hoathly and

Sharpthorne has suffered from persistent flooding problems as a result of

overland flow. Further north from here overland flow has been known to

cause flooding to the rear of Sussex Cottages (Sharpthorne). Any application

for development in these areas will need to consider the risk of flooding

posed by surface water run-off in a FRA.

The majority of this area is underlain by Wadhurst and Lower Grinstead Clay. The scope to utilise


form of SuDS. Evidence of the impermeable nature of the ground conditions in this area can be seen

by the numerous ponds in the locality. Permeable paving and other such systems can, however, be


Permeable paving and other such systems can, however, be utilised so they act as source control

and storage prior to a restricted discharge to a watercourse.

68


East Grinstead

(inc. Ashurst

Wood)

East Grinstead has few areas at risk from flooding. The main exception to

this is the area around Dunning’s Mill where there is an area of fluvial Flood

Zone 2 and 3 as well as a number of historical flood records associated with

the watercourse that runs through this locality. Any proposed development in

this part of the town should be accompanied by a FRA that considers this

issue. Another area of historical fluvial flooding can be found to the north of

East Court, including Lynton Park Avenue. Although no flood zones are

designated in this locality any proposed development should consider the

flood risk presented by the watercourse(s) in this area. Surface water

drainage will need to be carefully considered as part of any FRA undertaken.

For Ashurst Wood and the eastern part of East Grinstead the geology consists of sandstone,

siltstone and mudstone (Grinstead Clay) with poorly drained soils. The scope to utilise infiltration

systems will be minimal and therefore attenuation systems will be the most appropriate form of

SuDS. Permeable paving and other such systems can, however, be utilised so they act as source

control and storage prior to a restricted discharge to a watercourse. For the western part of East

Grinstead (and land to the west) the geology and soils should allow for infiltration SuDS to a certain

extent, probably in combination with an attenuation system, particularly for larger developments.

Land in this location overlies a minor aquifer and therefore high groundwater vulnerability will need

to be addressed in any SuDS design.

Crawley Down and

Turners Hill

For land to the north and south of Turners Hill flood risk from surface water

run-off will need to be considered. This is due to the topography of the area.

Otherwise, there are no specific flood risk issues in and around Turners Hill

that need to be considered in addition to current FRA guidance. For Crawley

Down, the area around the junction of Old Station Close, Station Road and

Grange Road is known to have flooded in the past as a result of surface

water drainage problems in the area. Any development within this locality will

need to demonstrate through a FRA that this surface water drainage

problem will not be exacerbated and where possible alleviated.

With regards to Turners Hill, the majority of the area is underlain by sandstone with moderately well

drained soils. This should provide scope to utilise infiltration systems as an appropriate form of

SuDS, although in some cases this may have to be combined with attenuation systems (particularly

for developments to the south of the village centre and for large-scale developments). The geology

in and around Crawley Down is sandstone with siltstone with poor to moderately drained soils. With

this in mind attenuation systems are going to be the most appropriate form of SuDS. Permeable

paving and other such systems can, however, be utilised so they act as source control and storage

prior to a restricted discharge to a watercourse. In both locations the land overlies a minor aquifer

and therefore high groundwater vulnerability will need to be addressed in any SuDS design.


Copthorne and

land east of

Crawley (within Mid

Sussex)

Fluvial flood risk is the main flood risk issue in this locality. Within Copthorne

and to the west of the village are significant areas of Flood Zone 2 and 3 that

are associated with a number of tributaries within the River Mole catchment.

Any proposed development within these areas will need to be accompanied

by a FRA. These aforementioned flood zone areas and other areas at risk

from all forms of flooding are also to the south of the village. Development in

this location will need to ensure that it does not compromise the ability to

implement the River Mole Flood Alleviation Strategy. Proposals for

development in the north west of Copthorne Common (outside Flood Zone 2

and 3, but has historically flooded) will need to be accompanied by a FRA,

which will need to address fluvial and pluvial flood risk.

Due to the risk of flooding in areas downstream (i.e. parts of Crawley and Horley, including Gatwick

Airport) care needs to be taken to avoid exacerbating downstream flood risk and therefore any

discharge of surface waters into watercourses in the area needs to be carefully controlled. Bearing

this in mind and the fact that the soils in the area are generally poor to moderately drained (as

demonstrated by the number of ponds and surface water ditches in the area) in most instances

attenuation type SuDS will need to be implemented. Permeable paving and other such systems can,

however, be utilised so they act as source control and storage prior to a restricted discharge to a

watercourse. Land in this location overlies a minor aquifer and therefore high groundwater

vulnerability will need to be addressed in any SuDS design.

69


Pease Pottage,

Handcross and

Warninglid (south

of Crawley)

No specific flood risk issues in and immediately around these three

settlements that need to be considered in addition to current FRA guidance.

Development to the south of Crawley will need to ensure that it does not

compromise the ability to implement the River Mole Flood Alleviation

Strategy.

The geology of all 3 of these settlements, and their surrounding areas, is sandstone and mudstone

with poorly drained soils. Evidence of the impermeable nature of the ground conditions in this area

can be seen by the numerous ponds in the locality. The most appropriate form of SuDS will be

attenuation systems. Permeable paving and other such systems can, however, be utilised so they

act as source control and storage prior to a restricted discharge to a watercourse. Land in this

location overlies a minor aquifer and therefore high groundwater vulnerability will need to be

addressed in any SuDS design.

Appendix D – Flood Risk Vulnerability Classification (according to “the Planning Practice Guidance on Flood Risk and Coastal Change”)

Essential Infrastructure

Essential transport infrastructure (including mass evacuation routes) which has to cross

the area at risk.

Essential utility infrastructure which has to be located in a flood risk area for operational

reasons, including electricity generating power stations and grid and primary substations;

and water treatment works that need to remain operational in times of flood.

Wind turbines.

Highly Vulnerable

Police stations, ambulance stations and fire stations and command centres and

telecommunications installations required to be operational during flooding.

Emergency dispersal points.

Basement dwellings.

Caravans, mobile homes and park homes intended for permanent residential use3.

Installations requiring hazardous substances consent. (Where there is a demonstrable

need to locate such installations for bulk storage of materials with port or other similar

facilities, or such installations with energy infrastructure or carbon capture and storage

installations, that require coastal or water-side locations, or need to be located in other

high flood risk areas, in these instances the facilities should be classified as “Essential

Infrastructure”).

More Vulnerable

Hospitals.

Residential institutions such as residential care homes, children’s homes, social services

homes, prisons and hostels.

Buildings used for dwelling houses, student halls of residence, drinking establishments,

nightclubs and hotels.

Non–residential uses for health services, nurseries and educational establishments.

Landfill and sites used for waste management facilities for hazardous waste.

Sites used for holiday or short-let caravans and camping, subject to a specific warning

and evacuation plan.

71

Less Vulnerable

Police, ambulance and fire stations which are not required to be operational during

flooding.

Buildings used for shops, financial, professional and other services, restaurants and

cafes, hot food takeaways, offices, general industry, storage and distribution, non–

residential institutions not included in “more vulnerable”, and assembly and leisure.

Land and buildings used for agriculture and forestry.

Waste treatment (except landfill and hazardous waste facilities).

Minerals working and processing (except for sand and gravel working).

Water treatment works which do not need to remain operational during times of flood.

Sewage treatment works (if adequate measures to control pollution and manage sewage

during flooding events are in place).

Water-Compatible Development

Flood control infrastructure.

Water transmission infrastructure and pumping stations.

Sewage transmission infrastructure and pumping stations.

Sand and gravel working.

Docks, marinas and wharves.

Navigation facilities.

Ministry of Defence defence installations.

Ship building, repairing and dismantling, dockside fish processing and refrigeration and

compatible activities requiring a waterside location.

Water-based recreation (excluding sleeping accommodation).

Lifeguard and coastguard stations.

Amenity open space, nature conservation and biodiversity, outdoor sports and recreation

and essential facilities such as changing rooms.

Essential ancillary sleeping or residential accommodation for staff required by uses in this

category, subject to a specific warning and evacuation plan.